The third base coach is responsible for many things. One of those responsibilities, under certain conditions, is signaling the batter to accomplish a certain event or chain of events based on his (third base coach) observations.
Now on some competitive clubs, there's usually a coach in the dugout of the opposing club who observes the third base coach very closely for certain signals and tendencies. For example, a third base coach may have the tendency to give the batter that you're facing the signal to "take a pitch." This can be his/her tendency every time a pitcher tosses two balls in a row, or, is having difficulty adjusting to the mound, or is just off for that game's appearance. I know a third base coach that worked with me was notorious for "take the pitch," for the current batter's first pitch every time a pitcher walked two batters ahead of him. After two games, at the beginning of every season, every opposing pitcher knew just when a to send a cream puff, and the moans and @#$! from our dugout could be heard everywhere.
If a club doesn't have a coach who observes the other club's third base coach - you do it. Since coaches in the amateur ranks tend to keep things simple and straight forward, watch for repetitive signals that the batter accepts and follows through with, based on his watching his third base coach.
Before I go any further, this can be a rather complex environment, so keep your observations and conclusions basic and simple. Don't try and memorize or rationalize. Remember, you're only looking for the "take the pitch" sign, nothing else. When you feel comfortable that the batter is going to take your next pitch, DON'T GET FANCY. Pitch you best stuff for a strike that you feel with give you that strike 99% of the time at that moment.