how can i tell if my dirtbike is seized?

i just got a 1999 suzuki rm250(im the third owner) and got it from the last owner for free, slips and all, but the motor might be siezed. what would be the easiest way to tell? when you try to kick it, the kicker dosnt budge, like there is a bolt stuck in the motor somewhere. also, if i were to put it in first, and go about 15(getting pulled by a car) the tire just locks up. also, i was going over the bike today, and it seems like there was pure gas in the tank(its a 2 stroke) tho the guy says that he knows for sure he mixed it. he changed all the fluids, im a little suspicious about the level in the tranny, but i doubt that would be it(that would just mess with the clutch). everything else on the bike is cherry, it just wont turn over, and i dont feel like pulling off the top end to find out( gaskets and whatnot....IM BROKE!!)

You've answered your own question. If the engine won't even move (like there is a bolt stuck in it somewhere) then it is seized up. The only thing to do is to disassemble the engine and see if it is salvageable or not. Take it from there. How much money do you want to sink into a"free"bike.

-Put the bike on a stand.-Put it in neutral. Make sure that the rear wheel turns freely.-Remove the L crankcase cover and try to turn the flywheel. You should be able to.-Remove the exhaust pipe and look in the exhaust port with a flashlight. Any scratches or scoring of the piston or cylinder wall and you need engine repair.