Forewarn

Full effect

When this Pokémon enters battle, it reveals the move with the highest base power known by any opposing Pokémon to all participating trainers. In the event of a tie, one is chosen at random. Moves without a listed base power are assigned one as follows: Power | Moves ----: | ----- 160 | One-hit KO moves: Fissure, Guillotine, Horn Drill, and Sheer Cold 120 | Counter moves: Counter, Metal Burst, and Mirror Coat 80 | Variable power or set damage: Crush Grip, Dragon Rage, Electro Ball, Endeavor, Final Gambit, Flail, Frustration, Grass Knot, Gyro Ball, Heat Crash, Heavy Slam, Hidden Power, Low Kick, Natural Gift, Night Shade, Psywave, Return, Reversal, Seismic Toss, Sonic Boom, Trump Card, and Wring Out 0 | Any such move not listed

In game description

When it enters a battle, the Pokémon can tell one of the moves an opposing Pokémon has.

Using Forewarn in Cobblemon

Abilities are passive and always active, so Forewarn costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.

See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.