Mountain caves have icicles hanging near the entrace. I always search from below so I can see the icicles rather than search from the peak.

Also after I clear a mountain, I check the map viewer just to make sure I didn't miss any. Some may consider this cheating, but I consider the devs making fenris hair so rare to be cheating as well, so all is balanced.

Frost caves are pretty easy to find though they have increased the difference in possible surroundings a bit with last big patch. But there are some things pretty consistent.
1. There is always an apron in front of the cave, usually roundish and always flat, so look for that from both above and below.
2. Most caves I have found face outwards from the center of the range or mountain.
3. The tops remind me of the tops of troll houses. One type is a bit more pointy at the center front with big icicles hanging down, easy to see from below, the other style is much rounder and really looks like snow trolls should live there. All caves have icicles hanging from the front.

I found one that was the top of the mountain which was so low there was no obsidian. It capped the mountain. But most are on higher mountains at or above obsidian level. If you are finding yourself skunked in areas you are pretty sure have them then use the seed generator to find them at first until you become familiar with terrain.

Watch a vid or two of folks making their way through caves as it is very easy to miss half of what is in them, and easy to be overwhelmed by just yoloing through them.
Travel safe.

Originally posted by The Masquerade:

so far ive only managed to find 2 dungeons in the four mountain bioms ive been in. havent come across any particularly large sections of mountains yet but compared to the loads of sunken crypts ive found in the few swamps ive been in. they also blend in as opposed to the bright green torches outside of crypts. im on my first playthrough and just killed moder. currently trying to get a set of the fenris armor crafted for that movement speed buff.

https://valheim-map.world/

Also I believe they spawn at around 200 m from sea level like silver ( I might be wrong with the number here)

But easy way to know if you are high enough on the mountain is to check if you see any obsidian stone, they usually are generated at the same height. So what I mean is if mountain has Obsidian on it, it may also have a Cave and silver, if no Obsidian, then its too small.

I did a whole bunch of caves for a Fenris set (plus upgrades) on the run I'm currently on.

Some things I noticed:

1) Caves are RNG and rare. Not every mountain has a cave.
2) Little weenie mountains never have caves. They seem to have the same height requirement as drakes.
3) They can be hard to spot but it's generally easier from above and below. I like to run around the whole mountain at the base looking up and inward, then climb up and look outward and down. I try to make sure I check the entire mountain at least twice from different angles.
4) It's easier to see the big mountains from the water. You can see the nice, big mountains from a long ways away but once you land your boat your view can be blocked by trees and terrain.
5) Don't be afraid to run through plains. You can kill deathsquitoes in swamp gear / food easily as long as they don't catch you completely flat-footed, avoid fulings and lox.
6) Parry stone golems with an upgraded silver shield then smash them with an iron mace. They're actually pretty easy once you get the hang of it. Don't use Frostner against them.

They are random. I ususally find the Moder boss stone & check out any mountain where Moder location is- Usually has a few more caves than others, tested on 3 worlds doing this same thing. You could try that.

Go out away from the center and you should find bigger mountains. You're not going to find nearly as many mountain caves as you do crypts. There's usually one or two per decent sized mountain, and you should get better at spotting them as you get more experience.

They have distinct look from top or bottom, a bit like a wolfs head. They are moderately more spaced than crypts or even eggs. I think they require similar height to silver so only on moderate high mountains or more. If the mountain range is thin, long, and has enough height you should be seeing caves along its length. Its pretty uncommon to not see caves on small mountains that have the height to support them so keep searching. Bigger mountains can have 6+ caves. I think ~3 is the average.

The cave entrance sometimes is easily obstructed from view so if there is a "bulge" type of peak/hill check around it, could be a cave facing weird direction.