Just hold a ruler against the chain- each pin should be 1/2" from the next on a new chain. On a worn chain, the pins will start creeping off the 1/2" marks, until at the end of the ruler (12"), the last pin will be offset from the last 1/2" mark by a fraction of an inch- that will be your chain wear. If it's more than 1/8", then the chain is starting to make your freewheel cogs wear out and form the "shark fin" tooth shape. Unfortunately, if the freewheel is worn, it will make a new chain wear very quickly, so you have to replace them in pairs when the chain gets too worn.

A new freewheel isn't very expensive- maybe $10- and a new chain is about the same. You need a special tool to remove the freewheel- if you have a bike cooperative, then they'll have the tool and show you how to use it. Then keep it all oiled well and the new chain and freewheel will last a lot longer.

But it sounds like your problem is more a rattling noise than a skip in the chain. The other posters addressed this pretty well.