The relay running the pump is probably the one closest to the pump terminals. To be really sure you have to remove the circuit board, turn it upside down and trace the copper path leading to the pump terminals.
The relays are soldered, very easy (for someone who knows what he is doing) to replace and relays are very cheap parts anyway, so if you go this way, just replace all of them.
If you do replace all of them, keep the old ones, they are not all bad and they might be used in an emergency or for testing reasons etc.
All of the above is valid if the pump relay is in fact sticking, but there are other causes that could result in a malfunction like the one you are having, even a bad (aka "cold") solder joint could cause something like that.
Anyway, relays are designed to be very cheap, they can last only so much time, and once you see the inside of them, you probably won't believe that we trust this cr@p part to run equipment worth hundreds or thousands times more.
In fact, for more serious applications, critical relays are placed on bases, so they can be removed / replaced quickly and easily, without soldering anything when they fail, and yes, we know for a fact that given enough time, they will fail.