Well with world war 2 they simply used planes to scout around bases and fleets, plus there's sonar or radar that could be used. Surface ships can be seen over quite a distance because the ocean is relatively flat the planes are high up.

If there were no planes around or it was earlier then a fleet would be sailing around with a number of light warships (sloops, corvettes, frigates, or maybe cruisers) out on its horizon watching for other ships within its own horizon and signalling the main fleet if they spot something (with flags before radios).

If we're really going back far to galley warfare then practically everyone is hugging the coasts and watch towers can see hostile ships and send signals.

Is it possible to go really far around or slip past a patrolling fleet, sure, if the ships have enough supplies to make it that distance and for sailing ships if the weather permits going through that course, or if the ship/fleet are really lucky and aren't spotted (which means sailing blind because the ship/fleet trying to slip through aren't going to know where the enemy is either unless they have their own ships/planes scouting around their fleet which are probably going to be spotted if they spot an enemy themselves) because it's not like there can be a solid line of warships blocking thousands of kilometres of ocean in a big line, but it's not particularly likely.