First time playing MOM, and when you put a unit on a cave, tower, ruin, etc. it tells you what is in that place, but sometimes I leave it be and forget later what was in that cave, tower, ruin etc. This is especially true if I come back to a game I started a few days ago.

Is there any way to look what is inside that cave, tower, ruin, etc. without sending another unit to re-explore the same place?

To get what your talking about you can use the Surveyor tool accessed via F1, or the info menu drop down, and hover pointer over a previously visited ruin, node, lair etc.

It is mainly used for selecting good Town Building locations.

Yep, F1 is what you want.

You can learn all sorts of useful things with F1.