JustinJay,
Just listen carefully and usually they will ask for the next ATC freq and then repeat it back to the ground controller. In my case, I have all the ATC UHF freqs for a 200mile radius from my location in my scanners. So, when they don't say their next freq, I usually catch them anyway.
Unless you have a setup like Ron's where he has all the UHF ATC frequencies programmed in.....the best way to hear the frequency change is to stay on the frequency they are currently on....when they are given a change of frequency --you will hear the change repeated by the crew.
If you continue scanning other frequencies that talk a lot you will miss the frequency change. The change occurs quickly and once it does (unless they cannot contact ATC on the next frequency) you will lose them.
Terry