How old is the bike? Did you store it outside? Is the chain rusty? Has it ever been tuned up? Do the brakes seem to work well? Are the brake pads worn down? Does it shift well or does it shift all by itself? Are there cracks (dry rot) on the tires?

If all that seems fine, put some lube (like tri-flow; not WD40) on the chain, pump up the tires, and go for a ride.