Look for signs of neglect/abuse. Rash on the frame/fairings/forks anywhere really. Look for oil leaks on the ground under the bike and on the engine. If it has fairings, feel around inside the bottom of the fairing for evidence of oil leaks and look at the parts of the engine you can see for any signs of leaking fluids. Check radiator hoses and connections for leaks. Check the fork seals and braking systems for leaks.
Make sure everything works, that the bike starts up easily and runs smoothly. Make sure the engine is cold when you get there to see the bike. You want to start it cold because if it is going to run rough it will run at its worst when it isn't up to normal operating temperature. Do not rev the engine while cold, if the owner starts doing this I would be very wary, this is not good for a motorcycle engine.
Ask about maintenance history, ownership history and when things like battery/tires have been replaced last. Do the tires have a lot of tread or are they worn? Factor in new tires when negotiating price. Motorcycle engines need more frequent maintenance than car engines do and the price of that maintenance can vary from bike to bike or even year to year along the same model of bike. Do your homework before you buy.
And last, I cannot stress this enough, do a vin check to see what the recorded history is and if it matches the information the owner gave you. I have a friend who was looking at used Ducatis earlier this year and 4 out of the first 6 bikes we looked at had mileage/stories that didn't match what the vin history told us. We had one guy tell us, "Oh, I bought it from my friend and it only has 7500 miles on it, I am the second owner." My friend asked for the vin number, ran a vin check, and found out that the guy selling it was really the 4th owner and that it was registered out of state and had changed ownership about a year prior with over 40,000 miles on it. We found another bike that was being sold at a Ducati dealership with mileage much lower than what was recorded at DMV when the bike changed hands 6 months prior.
Depending on the bike here are some common maintenance items and the cost to perform:
Oil change $75-150 (depending on bike/oil/where you take it for service)
Tires F/R $250-350 for both installed (could be as much as $500+ for high performance bikes or custom bikes)
Battery $100-150 (add about 1/2 hour labor to install for most bikes)
Valve Adjustment $300-600 (could be as much as $1200-1500 for Italian bikes)
New Chain $200 installed (add $200-300 for sprockets, if they are worn at all they should be replaced along with the chain)
If the bike has any damaged or rashed plastics these are very expensive (thousands of dollars) so unless you are adept at scouring ebay for used plastics or are going to install race plastics, which also are not cheap and come unfinished, you should probably walk away from a bike like that unless you want to make a streetfighter out of it. I've seen dealers selling used bikes with rashed up fairings/frames and parts hanging off them asking ridiculous amounts of money...
I would also steer clear of any bike with tons of mods on it.
BTW-30,000 miles on a motorcycle is considered pretty high mileage... even if the bike is 20 years old. That said, I'd take a clean bike with 30,000 miles, good maintenance history and meticulous ownership than a bike with no maintenance history and 15,000 miles that looks like it was left out in an alley for 10 years.