Similar with RDR1, you're just supposed to know the landmarks of the map sort of. They get progressively more difficult. I think it's easier to do ambiently throughout the story rather than all at once at the end as an 'endgame.' Most of the treasure areas are places you should pass through, OR they're just very, very distinctive spots.
For Jack Hall 2, the one that looks like a baseball diamond almost, it's very very very obvious when you ride by it. Like, I had Jack Hall Gang 2 in my inventory throughout most of the game because I did Jack Hall 1 around when I Was at the camp near by. But, I could never find it... I also assumed that it was outside of St Denis because it looked like mud flats, so I wandered around the muddy areas lookign for it... but nothing matched. FInally, I was randomly riding about in a region I hadn't ridden through, passed this spot that is unlike anything else in the game, and instantly recognized it from the map... and found the treasure immediately. I don't want to give it away because it's such an amazing spot in the game to explore. So, just a hint, if you travel near the railroads especially in the north western part of the map, you'll get close. The location is visible from the road, and when you ride past it you'll definitely recognize it. For more of a hint, the location is the game's version of a famous American natural landmark, or at least, a reimagining of a famous American natural landmark.
RDR2 treasure hunting is harder than RDR1 because the world is a lot more dense. There's one treasure map that has a drawing of ... like a Man on the Mountain type face on a rock cliff, and I assumed it would have been in the mountainous area of the map, so whenever I was in those areas I'd constantly be looking for a face-like thing on the edge of a cliff. But, that ended up being in a totally different spot and one that I rode by like 1,000 times but never looked at it with that perspective.
The treasure hunting is one of my favorite aspects of the RDR games. It's a component that I think they do so well, I wish they designed more of the game that way. I feel like they could have done outlaws the same way, or similar way. Like, you get a description of their last where about or description of where they probably are, and have to investigate it looking for them... instead of them just being a marker on your map. Things like treasure and the legendary animals are done in a good way, IMO, but others are just the total opposite.
