Might be a bit late if you're looking at it today, but anyway..
High milage generally means more wear...seems obvious, but what that means is that you will likely have to rebuild the engine to make it a good reliable rider. Although, I would do this if its got low mileage, with high mileage, you will end up replaceing more of the expensive internals.
On that note.. make sure the overall wear of the bike matches the what it says on the clock. If it's really tuckered out and it says its only gone 15,000 miles, than those were a rough 15k.
In general, look for things like: engine...is it loose and does it run.
frame....is it straight or modified
Wheels...are they round
,, check for loose spokes
Tank....dents and rust, old gas
Electrics..dont want hack jobs with lots of electrical tape
Forks...Pitting.. dont want that either
Also make sure the ownership paper checks out..This should actually be your first concern
Know what you want to spend and have an idea of what you want your bike to be when it's ready to ride. If the bike needs a little work and you plan to go all cafe on it, then a bike that's been neglected might not be a bad base.
If you want one to restore, make sure a lot of the little things are still there that may cost a lot or be hard to find to replace.
That should cover most things...
One thing that I do, and I'm not sure if anyone else does this is check all the switches on the handle bar. If they've been used a lot they will be kind of loose and the paint will be worn off the housings. If there reletively tight, then that's a sign of low milage. Speedo's can be rolled back and most other things can get replaced over time...but hardly anyone replaces those switches when they get loose
Matt.