I suggest trying to look it up first. There are plenty of resources on and off line for that kind of thing. Just see the previous posts.
Failing that, you could get a frequency counter, and try to get near them with it when they are transmitting. The only problem is that you have to be pretty close to get a good reading. I have one that can be set to remember a frequency when it gets a strong enough signal. Just keep it in your pocket, and when you see the guy transmitting, walk right by him as close as possible. Then pull out your counter, and make sure you got it.
Keep in mind repeater offsets. If they are not using a repeater, then the frequency from the counter is all you need. However, if the person is using a repeater, you only have the input, or the frequency used to transmit to the repeater. For scanner purposes, you usually want the repeater output, the frequency on which repeater retransmits the signals on. Listening to the output gives you a more consistent and usually stronger signal. If the frequency is near 150 MHz, expect the output to be 600 KHz above or below the input. For around 450 MHz, the offset is usually 5 MHz.