Are you talking about something like this
I haven't use this kind of row counter, but I believe you slide it up to the knob-end of your needle before you cast on, and then you just click at the end of each row (or every other row like Julie said above).
I have one of these
, and I just click it at the end of each row if I'm using a pattern with a lengthy repeat. Although, a lot of times, I just make a copy of my pattern & check off the rows as I go. Just depends on my mood, I guess!