The M24 video is simple RGB+I and the VGA adapter ignores the I signal. TTL levels on everything. One of these days, I'll work out a little circuit to use the I output and clamp the outputs to 1V p-p. But it works as-is for now.
So to use a scope, set 1 or 2 v/div, sync on auto, and adjust sweep rate so that you can see two pulses. You then can see the time between the leading (or falling) edges of any two consecutive pulses. The frequency is simply the reciprocal of that.
For example, let's suppose you've cranked the sweep rate to 5 milliseconds/div and you see two pulses. Looking between two consecutive leading pulse edges, you see 3 divisions plus a bit or about 16 msec. between the pulses. The frequency is then 1000 msec./second divided 16 msec. or about 62 Hz.