Hi there, I went to a couple of baby sign classes before having DD (years before! Lol) for work. I have used it with her from birth. She is 7 months now and recognises the signs I have used. She cries when I sign milk if she wants a booby feed, opens her mouth for more, cries at food if she is hungry, smiles and kicks her legs at bath etc, so really good indicators for me.
Kids who practice signing actually often end up with a "double" vocabulary, that is spoken words and sign that add up to more language than some others have at different age groups. It does not discourage them from talking smile they may use sign for some words for longer than you think is necessary but they will say it and lessen or drop the signing as they progress verbally.
Deaf children also often learn to spell earlier as they need to sign (spell) names out, also for eg. There is no sign for "egg" so they spell it grin clever little bunnies!
Good luck with it, just remember to be consistent!