When you go for a massive mercenary push with the Germans, which is not that uncommon strategy recently, make sure you group your Explorer with your army to prevent optional opponent's Spy ambush. Spies, as it's widely known, are a great counter to mercenaries, but their main weakness is an enemy's ability to kite. If you manage to get them in melee combat, they become very scary, but German mercenary army, mostly consisting of Jaegers and Black Riders, is ranged. Your opponent then may try to catch it by going stealth with his spies, with your explorer near your troops, though, it won't be possible, as he detects them nicely and early.
We all know how advantageous additional line of sight of a French Native Scout can become. Players often move them next to enemy's military buildings to quickly get a knowledge of opponent's upcoming reinforcements and army composition. There's one easy trick to prevent this. When the dust settles and you have some spare APM, you can move your explorer unit around your military building to try to find that cheeky Frenchie. Make sure you have some army to quickly kill him, as your enemy will instantly receive a notification of his Native Scout being spotted and will certainly try to run him away. It's a pretty useful action that will not cost you anything, as French players tend to heavily rely on him during the games.
If your explorer is currently unavailable, you can always use an Envoy or your own Native Scout, if you happen to have one, to do that, as they see stealth as well. The same goes with Spies and Ninjas. Watch out for treasures got by your enemy, as he can get a Native Scout himself β remember then to use this simple trick!
If you like the tip, like the post! Let me know whether you find it helpful so that I can improve my advice in the future!
EDIT (9th of October):
The summary with all units and buildings with a βSees stealth tag":
Spies, Ninjas, Envoys, Explorers/War Chiefs/Heroes, Native Scout, Shinobis; Town Centers, Agra Forts, European Forts.
Side note: although it may appear that most units that can go stealth can also see other units in such a stance, it's not always the case Troops like Forest Prowlers, Tashunke Prowlers, Jaguar Prowlers, Coyote Runners and Navajo Riflemen do not have the aforementioned tag. The same applies to other buildings which you'd most likely suspect, so generally the ones that can fire. They do not detect stealth armies, don't be afraid then to come underneath a Blockhouse or a War Hut with your Native Scout!
βAnti-stealth range" β edit (5th of January, update: 14th of May):
Kaiserklein wrote:Btw it would be cool to know what the range of the "anti stealth" is. I think it's around 1/3 of the LOS of the building and probs a bit more (2/3 ?) for units
It depends on the building or the unit, although some common patterns may be found. A Town Center detects stealth units in around-24-tile radius, with 32 attack range; a Fort does it from 26 tiles away β equally to its attack range; an Explorer and Heroes with 18 base LOS and 16 range seems to do so within 16β18 range; the Agra Fort β around 6 tiles closer than its maximum range of 26; a Spy β about 20 tiles away with base LOS of 28; Shinobis and Native Scouts β the same as their base LOS: 18 and 22, respectively; Envoys β within more than 20-tile radius with their base LOS of 26.
Proof pictures β shows two units of the same type, with the distance in between them being equal to their line of sight, and a Spy being detected in the place he stands (hence: anti-stealth range):
[spoiler=Spies with 28 LOS][/spoiler]
[spoiler=TCs with 32 attack range][/spoiler]
[spoiler=Envoys with 26 LOS][/spoiler]