if you can find a number on the belt then just go ask for one of those.
If not just take it off and take it into the store...
failing that the belt numbers are generally of the form QPKRXYZ where Q is the number of ribs the belt has and RXYZ is the length of the belt in millemetres. eg 4PK0990 would be a 4 ribbed belt 990mm long or it may be length divided by ten I can't remember...
Pray to god, but row to shore...