He he
To be a bit boring and give a more pratical answer...I think established wisdom is:
they can tell you they are doing (or even better) are GOING to do a wee/poo
they are showing some interest in potty/pants/toilet
and can perform on demand at least occassionally.
Having said that my son wasn't doing the first when I potty trained - I reckoned he just wasn't going to tell me until he could see there was an implication of doing domething, iykwim...In retrospect, night have had an easier time if I'd waited...