Look for clues of leaks: coloured crusty deposits on parts of the engine bay, or patches on the ground where you park up. Does the area around the rad cap look clean? If not maybe the rad cap isn't holding pressure any more and the coolant is boiling off from there.
If you're losing coolant, there's 3 possibilities:
1) there's a leak in the system where liquid coolant is coming out. -- look for tell-tale coloured leak stains. Garages can do pressure tests of the cooling system to find hard-to-spot leaks.
2) the coolant is boiling off. -- I think this would only occur from the rad cap
3) head gasket failure -- the coolant is leaking into the engine oil (or outside, but I covered that in point 1) -- look for "mayonnaise" on the dipstick or around the oil filler cap.