There are ways to tell when your skis need a tune. First, flip the skis over and look at the bases. This means the wax is gone and your bases are more brittle to damage and wear out faster. Now check the edges. Slowly and lightly, as to not to cut yourself, run your finger down the edges.
Chances are they are burred and dull.
You can also hold a true bar over the bottom of your skis to see if they have become concave. If you see light under the bar in the middle of the ski, this could mean you are skiing on your edges instead of the full bottom of the ski making it difficult to ski on. Advanced to expert skiers usually tune their skis after 5-7 times of skiing. They want the edges to be sharp for control and the bottoms to be waxed for speed and ease of turning. Intermediate to advance skiers should tune their skis after 8-12 days of skiing.