Books reprice quickly on sharp action and slowly on public action - a line that jumps on modest volume tells you who bet, not just how much. The classic tell is reverse line movement: the line moves against the side receiving most public bets, implying respected money on the other side. (“Action” also has a grading sense: an “action” bet stands even if conditions change, e.g. a listed pitcher is scratched.)
Odds feeds don't carry ticket counts, but action leaves tracks in prices: velocity and cross-book agreement of a move are observable in ParlayAPI's timestamped odds, and the movers view ranks where the market is moving hardest right now - the standard proxy for where serious action is landing.