Almost all my summer caught twenties have been located because of ducks taking fright. My theory being that it is only bigger pike that eat ducks. Take a note of where other anglers are catching bream. Bream have fixed patrol routes. If these go near that heron you know you will be onto a bigger fish.
Cormorants are another giveaway, they feed on what pike feed on, simple.
Watch, as the tide drops. There is a magic moment when the receding tide triggers a pike into moving. The reeds part as they leave their lair. Another, literally stirring giveaway, is rowing along a shallow margin, watching for tail swirls in the mud. Without warning, all around you, sometimes more than can be counted, swirls of mud.