There's usually a number somewhere on the pot (look around the "collar" opposite the leads). It will be something like 102.
The first two digits are the "significant" digits. These are the first two numbers in the value.
The last digit is the "exponent". This is the number of zeros after the "significant" digits. The resulting value is in ohms.
So, for my example of "102", that's a 1000ohm or 1k ohm (k = kilo = 1000) pot.