How many bikes have you checked out/bought in the past?
1. Ask for the service history...if the guy doesn't take it to the dealer for this...ask for his log. If he doesent keep one, that is one nock agains the bike...no records means no proof of maintenance...not always bad, but not a positive. Check the VIN and see if this is a California model or not (will have evap canister on the bike as well...important iif you live in Cali I imagine. Write down the VIN and I think you can ask your insurance company or the dealer to see the history of the bike (if it has been crashed--reported--or not).
2. Bike should be COLD when you walk up...if not, then he either jumped it or got it started and warmed up so it would start easier (the first time)...this isn't normally a problem with an FI bike, more often with bikes that have carbs...but still...it is a potential sign.
3. Check the color of the oil level and color of the oil through side viewing window (rt side engine case ahead of the clutch housing) with bike on center stand...oil level should be between two lines--if too low, take a note...will probably be darker as well. If too high--above the second line -- that's definitely not good either.
4. Bring some hex wrenches and peel off the left side fairing (ask first) and look at the coolant level, cold. It should have coolant in between the two lines. After you start it up and get it to temp it should rise somewhat.
5. if you have a Volt Meter, take the seat off. Check to see if hte factory tools are all there...and check the cold voltage on the battery. >12.2V but that's not enough. After you start the bike the voltage (DC) should be >13.5 at 2500 rpm and less than 14.8V at 5000 rpm. Here's the fault finding guide link from electro-sport...https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCQQFjAAahUKEwj3nqCV5PbHAhXGFx4KHYSXC3Q&url=https%3A%2F%2Fwww.electrosport.com%2Fmedia%2Fpdf%2Ffault-finding-diagram.pdf&usg=AFQjCNEEc6HtTj-o-40A7DBmSuZqLu0vNg&sig2=whW7UIdIsdJpmv64aetMlw&bvm=bv.102537793,d.dmo
6. Let the bike warm up at idle for at least 5 minutes...the temperature should be >175F...if its a hot day let it idle for 20-25 minutes and the temp should go up to 220...the fan should kick on...if temp goes above 225 and no fan you have an issue (the displayed temp is wrong or the fan switch is not working or there is an open circuit in the fan circuit...)..either way you need to get the bike to temp and make sure the fan comes on...you can rev the bike safely after 5 minutes and it is up to temp to get the temps up...
7. if it is at temp, check the weep hole under the water pump to see if any coolant is coming out...use a flashlight, see if the weep hole is not gunked up first...if it is, un-plug it with something small...if there is fluid coming out the shaft seal on the water pump is shot...this is not good but repairable.
8. You should also ask for a test ride--if possible.
9. Look for crash damage to the fairings...cracks or obvious repairs...better if you can pull off the side fairings and check them and the seat off to see if the rear has been repaired. all the fairings should line up easily...all the fasteners should be there...and should be factory...even behind the front wheel in front of the front cylinders...these little clips are a pain, sometimes we replace them with similar types...not too concerning, but they should all be there.
10. Look at the engine cases to see if they have been ground down. Look for oil leaks/fluid leaks under the bike. Look at the oil plug and oil filter see if there's any leaking oil.
11. look at the bar ends...are they stock or aftermarket...if aftermarket ask to see the factory ones...they should not be ground down...a scuff is OK, ground down means laid down...not just a tip over. Look for dents in the tank...Look at the front turn signals...cracks around them means they were either crashed and broken or tipped over. If they are replaced with aftermarket, ask to see the stock ones ("in case you want to put them back on") if they are scuffed or not there they were probably bnroken in a crash.
12. with bike on center stand and facing a garage door, check out the headlight pattern...I've seen it where one was higher than the other...menaing the front fairing stays were bent ...you should be able to see this also if the front windscreen and fairing are closer to one handlebar or the other...as you sit on the bke with the front wheel straight ahead.
13. Bring a string...and do the alignment method on the front and rear wheels...with a SSA if these are not aligned, the frame is bent (http://www.motorcyclistonline.com/simple-motorcycle-wheel-alignment)
14. Look at the chain and rear sproket...if the teeth are worn, ask when it was replaced...if no records assume you need to replace front, rear and chain....this is at least $200 negotiating point. Same with tires, used tires are OK, new better, if they are at the wear bars, it is a negotiating point worth at least $300 to $400.
15. Look for rust at any of the subframe welds...to me this indicates it was bent, cracked the paint and is now weakened.
16. Look under the seat and see if he has any switched relays for accessories...is there a fuse for the relay? How are the electrical connections, are they professionally done or look like an amateur slice and dice job?
17. Look at the brake fluid color and the clutch fluid color...take the tops off and see if there's gunk in the resevoir...should be light or color of honey at most...brown is bad, gray or black is horrible. front and rear rotors should show some wear but not big ridges or gouges...a little rust is OK if it sits outside, but should wipe off ...lots of pitting is bad. The pads front and rear should be able to be inspected with a flashlight...should have more than a 16th and near an 8th at least...if it looks like its almost flat means they are way gone...need not only replaced, but you need to check the caliper bores as well and maybe refresh. Have someone push down on the rear of the bike and get hte front wheel off the ground...spin it. it should spin freely, you should hear the brake pads lightly sing on the rotors, but it should be fairly even and very light...wheel should spin a few rotations...not stop quickly or hear the singing pulse. if it sings, means the rotor carriers are bent most likely, an indication of a crash or someone was hamfisted changing the front tire...not a good sign. same with the rear wheel and bike in neutral, cept it won't spin as much because of hte chain drag...so listen to the rear brake for dragging caliper...should not drag too much...and have some meat on the pads.
Plenty more , but that's some of what I try to go over...if you can get a test ride there's more to do/feel...but I would ride it without earplugs...you should hear the Pair system flapper opening and closing when you start out and come to a stop...the chain should sound smooth when idling and clutch in...taking off shouldn't be snatchy. It should snick into second gear, third etc...easily. braking should be straight..