My DD has been quite slow developing in some areas but very fast in others.
She is 19 months old and has started walking (more than crawling) in the last week or so. She says a couple of words but not many.
However, she is very quick at understanding instructions. Eg, DH yesterday told her to go into the lounge and get her new dolly so that her dolly could ride in the pram. Off she went to the lounge (on her wobbly little legs - cute!), got her dolly and came and put it in the pram. It took her so long that we had no thought that she was actually doing it, until she brought the dolly to us! Bless her. She also points at everything, knows all her animal noises, can pick out cards from a pack 24 picture cards if we ask her to pick one out and plays pairs with her animal cards.
So, onto the reason for my post. For the last few months, when I have told DD that I am going for a wee, she follows me into the bathroom, pulls out her potty from under the sink and sits down on it (although getting onto it has been an issue because she's quite small so tends to have to put her foot in first and then sit down! 
). She's got her nappy on at this point and I haven't tried taking it off yet because I don't know if she's really understanding or not. What do you experts think?
She did do one wee on the potty ages ago, pure fluke, we'd just bought it and I put her straight on after her lunch one day (she was about 14 months old). I made a real fuss of her, but I think she thought it was because she'd sat on it, not because she'd wee'd because after that she would clap herself everytime she sat on it - very funny!!
So, should we try or are there specific signs that we should be looking out for?