Once, when I was a kid, my father explained how to tell the difference between the cold tap and the hot tap when they're not labelled. The cold usually comes straight from the mains so has high mains pressure whereas the hot is usually from a cistern in the attic so has the low pressure of a head of water. So just test the pressure of each tap by trying to stem the flow with a finger, or simply by looking. What we want, energetically, is to tap into the fresh mains rather than develop a stagnant head.