Rare it is, that the chain would be too tight.
Pick a spot between the sprockets and push down on it, if there is less than 3/4" of give, yer fine.
Best way to retension your chain is to loosen both rear wheel nuts, and then sit behind the moped and push down on the tensioners (those little bits of metal under the rear wheel nuts) with your thumbs, while eyeballing the rear wheel to make sure it's aligned properly, and not leaning off to one side or the other.
Once you get it where you want it, spin the nuts tight with your fingers, and then put a wrench to them, making sure the wheel alignment remains as you put it.
Rather simple, all in all.