My thought on this is that you cue and/or sign when they're going and when they're on the potty, and wait for them developmentally to cue or sign to you when they have to go.
I cue all the time (meaning, when I'm holding her over the potty, and when she actually is going) and sign if my hands are somewhat free and I think of it. Just before 9 months of age, my dd made a "th" sound while she was peeing, every so often. She even did it one time right before peeing. Just the other day, she made a "s" sound while sitting on the potty, but didn't pee. Then the next day (she's a year old) she made a "s" sound to me, indicating that she had to pee, did it a few minutes later for a poo, then did it again a few minutes later for another pee.
If I were quicker with signs, I would sign when I saw (or felt) her peeing, wherever she was. I do still cue her on those occasions.