Check is signed just a bit like bill, so be careful not to get the two confused.
To sign check, hold your non-dominant hand open and horizontally before you, palm facing in and thumb sticking up. Then take your dominant hand, form it into the ASL letter C hand, and use it to grasp your non-dominant hand, starting from the curve at the base of the thumb and sliding to the edge of the index finger. It is like you are tracing the top and bottom edges of a check.
Teach your toddler the check sign when they begin to show curiosity towards you or your spouse handling checks, when organizer your billfold at home, or when you are buying something and paying using a check. Eventually, there will be a good opportunity to start your kids young and already make them aware of the value of money, saving for a rainy day, and making mindful purchases.
This is a useful flash card taught best in conjunction with actual purchases, or when your toddler sees play money or play checks at home.