A sharpie is a good start for those new to base edge filing. Another way to tell is to look at the base of the ski and also the area of the file that is cutting. There should only be filings--no black base material--in your file. You can check for this when you clean your file which should be often--as in several times during the filing process. If you look closely at the base edge itself you will be able to see where the file has cut and where it has not. It should cut from the outside of the edge to inside assuming your bases are totally flat...
Typically base edge filing takes little pressure with one thumb directly over the file right where it sits in the guide and the other thumb directly over the file right where it sits on the edge. Pressure should be light and downward--take care not to bend the file! This can create a "rounded" base edge and/or over-beveling.
Start with your finest, shallowest toothed, flattest, sharpest file. Make shorter overlaps for the first pass, gradually increasing the overlap for subsequent passes. The whole process shouldn't take more than 4 passes or so--finish your last pass with a full length pass of the file.