I happen to have just gotten off the phone on a snowy Monday morning (brrr!) with a mom inquiring about WeeHands classes.  She knows her baby is ready to start signing because her daughter is pointing to objects and looking at her parents expectantly while sometimes vocalizing.  You can without doubt sign to a baby before he or she starts to use gestures and to point but these gestures are a good sign that babies are ready to express themselves through sign.  A baby will understand what you sign to them before baby starts to gesture, point and to use signs. Understanding a new language always comes before being able to expressively use that language.