You use your map of course. Like you would in real life.
Sometimes it makes sense that you know the exact location that you are looking for (like if a NPC tells you to go somewhere and would realisitcally be able to pinpoint that on a map), but if you are looking for a lost tomb, or ancient ruins, then it would make more sense that you have only some vague clues as to where it is, like in what area or near what known location, and have to explore to find it. Right now the only two options are knowing exactly where it is, or no clue at all.
I would also prefer if the in-game map wasn't this 3D-photo of Skyrim, but rather a drawn map, like the ones you can find on tables, where scale, features and locations may not be so exact and accurate. The layout and characteristics of Skyrim aren't so difficult to learn (because of the amazing and detailed work the landscape and terrain designers put into it), without map markers and quest markers you could instead build adventures around the knowledge of Skyrim that we pick up during the game which is a better mechanic, just like a novel works better if the parts of it tie into the understanding of the environment, personality and motives of the characters that we get to know.