Depends on the field.

In many club fields, there are frequency boards where pilots hang clips, or cards, or what have you...or perhaps take a clothespin from the board, and put it on their transmitter. In other places, there are radio impounds, or such methods.

At other smaller, or private areas, there's frequently nothing so fancy. In that case, many of the guys simply KNOW what their buddies fly on which channels.

In EITHER event, it's always struck me that there were 2 very basic rules of thumb for frequency management:

1) ADVERTISE YOUR CHANNEL. I have channel flags on my radio, have my channel written on both sides of my hat, and have a channel number on each of my aircraft. Heck, if there was a Nashville Predator whose jersey number was 48, I'd probably WEAR it at the field. Point being, put your channel number everywhere you can think of to maximize the possibility that an observer will KNOW what channel you're on.

2) ASK! Regardless of which (if any) of the methods above a field may use to manage frequencies, if I'm going to be operating with another hobbyist nearby, I will ASK THEM what channel they're on. Believe me, NOBODY will mind. I do this EVEN if they're displaying their number as I described above...just to be certain they didn't, say, buy a new radio yesterday, and haven't changed their hat yet.

3) (I know, I said 2, but 3 is just something I do extra...probably borders on obsessive, but hey) Any time someone's in the air or has a plane running anywhere, when I'm ready to turn on my radio, I always WATCH their aircraft as I power up...JUST IN CASE. I still do this even when I'm flying with, say, shakes268. We've flown together quite a few times at this point, I KNOW what channel he's on, we've NEVER had issues before, and there's just no reason to even IMMAGINE we might. But I do it anyway.

Thus endeth gb's obsessive tirade for the day.