Kids may show signs of understanding the words used with toileting, understanding how to use the potty and even wanting to sit on it, asking to be changed frequently, and trying to hold it some. They may have dry diapers for longer periods and may go off on their own behind the furniture for privacy. There are books that you can read to them and they may even want to push the issue. When they ask to be changed, don't necessarily drop everything -- tell them you'll do it as soon as you can. The sensation of a dirty or wet diaper can reinforce independent toileting.
Most importantly, be patient! Children should understand that it's natural -- everyone does it. Adults or older siblings can model positive behavior. Toileting should always be a pleasant experience. Be sure to encourage and praise them repeatedly, give them rewards for success and never punish or reprimand. Let them believe they are in control, and the toileting will happen sooner rather than later.