Looks like a well cared for bike.
You'll need to look at the normal stuff, just like when buying a used car. Tires, brakes, battery, oil, etc. Some stuff may need to be changed just due to age (cracking fork boots, etc).
If the current owner doesn't know when the last time the sprockets and chain has been changed, I would swap them out for new ones so you know how many miles they have on them.
Make sure all the lights work. Pop off the right side cover and make sure all the wires are there and nothing has been cut/spliced poorly.
You can take off the left side cover and check the air filter.
Put the bike up on a stand and shake the front forks and handle bars, making sure there is no slop or binding. Also, see if the swingarm doesn't move side to side. Spin the front and rear wheels to see if there are any funny noises. Do the wheels spin freely?
The fork oil may need to be changed if the current owner doesn't know when he did it last.
There are many more things to check. Don't be afraid to take off the seat and look at the battery and wires. Get down on your knees and look at the engine covers, making sure they are not cracked and if they are, make sure they have been repaired correctly.
Look into the gas tank, any rust?
Does the petcock turn easily and stop the flow of fuel?