Here's the checklist I use when inspecting a trike:
Start with the overall condition, does it look like it hasn't been washed since 1985? any obvious damage to the plastic, seat, tank, and handlebars/controls will clue you in to possible accident damage. Check the condition of the tires. Next check the front and rear axle bearings and the swing arm bearings, try to rock the machine side to side and note if the axles are staying put while the rest of the frame moves a good distance side to side, is it making clunking noises while your doing this? check the condition of the chain and sprockets, is there too much slack? has it been lubricated or is it dry as a bone and rusty? in the case of a shaft drive machine carry an adjustable wrench and pull off the filler cap of the final drive, is it full of rusty water/white foam? or is it fresh oil? Check the engine oil, is it clean or full of water/white foam, does it smell like gas? Pop open the airbox, is there a foam element in there, what condition is it in? Test the front and rear brakes, do they feel mushy or do they have good pressure? fire it up and look for excess smoke out the exhaust, check for engine noises while you vary the throttle speed, does it idle or keep stalling out? Once you are sure the brakes work take it for a short ride, note any wobbles from bent axles/wheels. Does it pull to one side or the other? (bent forks, frame, or uneven tire pressure) check the operation of all the controls/ lights, etc.
Once you make your way through this list you should have a really good idea of the real condition of the machine, even if it's just been cleaned and shined up to try to hide it's flaws. Then it's up to you to decide if you want to get involved with any repairs it might need and if it's worth the money to you.
If I can't fix it, it ain't broke!!