I often come across clues saying that engineers are good for detecting mine fields but how does it work? When I go near a mine field with my engineer, trying to find a path across it, nothing happens. Which buttons do I need to press? Or what should I do? Help really appreciated.

You should be able to see them on the floor - they appear as a chequered pattern of yellow and black. It should happen automatically, you don't need to press anything. You can only see the mines if you're standing in the square next to them, either adjacent or diagonally.

If you're doing multiplayer, then only engineers and paratroopers can see mines I think.