Get some elsan blue fluid, which doesn't affect rubber or so they say, put a few drops of the fluid in the recess under the cap, put the cap back on.
The valve will have to be at the bottom of the wheel.
Also remember about it when you check your tyre pressures.....:doh:doh
When they press in the valve they will be covered in blue and as you know, this is difficult to get off your hands.:augie
Or try food colouring, or ink jet ink, you could have a different colour per wheel......:naughty