The reason I suggested checking the coolant level cold is that when it's hot, it expands and you have a higher coolant level, if you check it cold and the level is lower, you can add coolant.
Yes you can put a dye into the coolant and check foer leaks with a black light ( and yellow sunglasses). With the dye, even if you ccan't see a leak it's easier to see the stains where the leak was before it dried up.

If it's not running hot, just make sure to carry a jug of coolant with you like junkmonkey suggested.

 

Have you checked the raditor cap for pressure??? This could be your problem...