Bowel control is often achieved before bladder control. Parents can usually tell when when a bowel movement, or BM, is about to happen. When they suspect a BM is imminent, they can bring their child to the potty to see if the child is able to relieve themselves there. However, potty training is a very individual process, and some children may achieve bladder control before bowel control. Ultimately, you'll find what works best for your child.
At some point your child will no longer have bowel movements during the night. They become a daytime process for most children. The more regular your child is, the easier it will be to start bowel-control training. Children often stop playing when they are having a BM. Some even hide in a favorite spot. Others just grunt or get red in the face. Saying, "I see you're having a BM" helps a child identify what is occurring and associate those sensations with the process.
Constipation, if part of your child's history, may actually delay toilet training for your child. This is not anything to be worried about, but just one more variable you can be sensitive to.
Explain to your child that you will be taking him or her to the toilet or the potty chair and that you expect the bowel movement to go there rather than in diapers. "This is what Mommy and Daddy do, what grown-ups do and now what you should do because you're getting so big and grown up" is the kind of language to use. Give advance notice as to when this procedure will start. Children need to hear what you expect in pleasant tones and words. They can't read your mind. Your attitude, your anticipation, your relaxed tones will also convey much of the message.