You know when it starts to feel easier! I know that sounds really simplistic, but once you don't find it tough and aren't feeling like your arms are about to give way on that last rep, then it's time to go up!
Don't expect to be able to do as many reps when you first up the weight, which can be a bit frustrating. And if part way through the sets it just gets impossible with the higher weight, let it drop back to what you were lifting previously to finish off.