The best way is usually to look at the console that opens up when you start the game. Red print means an error occured and the mod will be skipped.

Because there is no actual integrated mod support it solely depends on how the modder implemented it. Like you'll be able to see valheim plus cause it changes the logo. You'll see epic loot cause there are additional info tabs in game. But smaller changes or less documented ones are gonna be pretty hard unless you check if they're doing what they're supposed to do...

But theoretically all mods that are loaded should show up in the console.