There are several places throughout the game where you will notice a Power number listed next to a symbol of crossed swords. This is a rough estimate of the strength you have, can gain, or will need to defeat an opponent.
Keep in mind that Power is only an estimate, so it does not take into account things like advantages/disadvantages from different troop matchups, or the current buffs or boosts you and your opponent have. Using these outside factors effectively can allow you to defeat an opponent that has a higher power rating.
When choosing an opponent, always check their power against the power of the march you are sending into battle.