Without knowing it, many pitchers, if you look hard enough, will tell you what they are going to throw. It could be something they do with their glove and/or their delivery. It could be a hundred other things as well. At the higher levels, especially with video analysis, it becomes very hard to prevent the other teams from finding out if pitchers are tipping off their pitches.
The video below will show you six ways pitchers commonly tip off pitches to batters . If you are a pitcher, make sure you are not doing them. Ask a teammate or coach to watch you pitch to see if they can pick anything up. If you are a hitter, look for these things and anything else that pitchers may be doing that can help you determine what pitch is coming.