Detecting Hidden can be used to reveal hidden players and traps. There is both a passive and active form of Detect Hidden. When invoking this skill (active use), the character is given a targeting reticule and asked, "Where will you search?" The skill check is run when an area is selected. If successful, hidden characters and traps within the search range will be revealed. The further a hidden character or item is from the targeted tile, the less chance to detect successfully. A stealther may re-hide immediately upon detection, wherein Tracking should then be used.
There is a 6 second delay between uses of Detecting Hidden. The passive Detect Hidden range against a character using Stealth is a 4 tile radius; stealthers are advised to stay at least 4 tiles away from others. Detect Hidden works against both Invisibility and Stealth, though not equally. Invisible characters are easier to detect. Use of the Detecting Hidden skill in a home the character owns or is friended to is always 100% successful.
You will need to have tinkers of different skill levels create trapped chests for you. For starting off after training with an NPC have a tinker that makes trapped boxes for breaking paralysis make a trapped box for you. The tinker should have a skill level around 35. This will take you to roughly 60 detect hidden, at which point you can start using a trapped box made by a GM tinker. For faster skill gains you can allow a Satyr to use the Discordance skill on your character in the Twisted Weald once you reach around 50. This will allow you to use the lower level trapped box for much longer.
With the classic client create a single button macro to use the skill on your last target. Manually use the Detect Hidden skill on the box once, then continuously press the macro button to continue training. For the enhanced client create a new hotbar and drag the Detect Hidden skill icon to it. Then place the box on the ground if you are not using the enhanced container option, or you can leave it in your pack if you are using the enhanced container option. Then right click the icon on the hotbar, choose target, and choose "stored" and target the box. After that you can use that macro to train your skill, but you cannot do this inside a house you own, co-own, or are friended to.
Join a faction and have a bunch of enemy faction traps placed next to each other. This method can be extremely quick for gains, but is difficult to setup. If you don't get any gains, reveal players until you do and then go back to the traps.