How to know? Sometimes they whistle at you! My truck would occasionally get a wheel squeak (I thought), usually after going offroad, so I thought the brakes had a little dust or a gravel in them. The squeak would happen for only a partial wheel rotation. But then one day I realized the squeak only happened when the CTIS was pressurizing, and usually in cooler weather! The left front has a CTiS seal leak, but both fronts have new seals from a diff upgrade so not worn out. CTIS may or may not achieve HWY mode, failing with either all 5 blinking lights, or with HWY and EMER lights on together. Later I noticed that if there was plenty of air available the CTIS would work every time (as with shop air hooked up). No doubt the shop compressor could overpower the leak.
Haven't tried it but one day I want to add a much larger air tank in parallel with wet tank to assist the compressor. Could be handy for air tool use also.