Wait to put on your helmet or ear plugs. After the motorcycle is started, listen closely to both the top and bottom of the engine. If you pull in the clutch and there are unexplained rattles near a transmission with a wet clutch, for instance, there may be a problem. Your previous research should help you here as well. Every motorcycle has some idiosyncrasies; this starting procedure and the ride are where you should pay attention to them.
If the bike has a steering damper, turn it until it disengages. Turn the bars from side to side and make sure there are no rough spots in its movement. Make sure the levers and pedals can be reached easily and ask to adjust anything that could pose a serious hazard. Also, if mirrors are present, check that you can use them best to be safe on someone elses bike.
Ensure that you know where you are and where you are allowed to go on the test ride. If you are unfamiliar with the area, the owner should be able to point you in the right direction.
While riding, make sure to change through all gears. Notice any issues with the shift action or if it feels severely loose.
Check that finding Neutral is easy and positive. Some bikes are easier than others, so make a mental note to ask the owner later if you have an issue.
Get on the gas hard in each gear, as permissible, and get a feel for the acceleration. Make sure the bike remains in gear, the power delivery is smooth and interruption free, and that the bike tracks straight it should steer easily and not veer to one side if you loosen your grip at the handlebars.
Try the brakes and ensure that they have a very smooth and progressive feel. Binding and/or pulsing could be indicators of damage to rotors or even problems with the hydraulic system.
When decelerating, make sure that the bike remains in gear and does not have a tendency to jump out between your second and third cogs.