Welcome to the forums and PBW, always good with some fresh meat
If the ship is cloaked (cloaking on), you will see a dashed circle around it. Many players, myself included, often add the stealth armor to the designs for the combat bonuses and not for the stealth ability.
Beware though; Even if your opponent have left that one ship uncloaked, there can still be a cloaked fleet hiding under the ship you can see. If you attack, all ships in the sector will engage, cloaked or uncloaked.
Also remember; Cloaked ships don’t sweep mines. The best way to insure that someone don’t sneak a cloaked fleet behind your defences is to drop a few mines at all warppoits.