How to determine accuracy of multimeter?
The simple answer is either that you cannot or it's easy. It depends on your definition of "cheap". The difference that you cite is more than 4%, which is more than you should be dealing with. At least one of those meters is badly off.

What you need is a reference voltage.A good starting point would be the LT1236, which should cost you about 4 dollars plus shipping from Digikey. This will give you a 5 volt output with an accuracy of 0.1%. If this is too much for you, a TL431 will cost 40 cents, but its accuracy will only be 1%. There are lots of other parts which will do the same thing, and lots of suppliers, so get on the web and do some research. And just as a bit of advice, you're better off with a 0.1% unit.

An alternative would be to find someone with a good meter and ask to use it to check yours.