That's what the big red light is for.
But actually I remember what I did in the past was press record in advance and put it on pause, then unpause when ready to tape.
Well, that's the only way to start recording on the M10. REC puts it into record mode with pause flashing, and you unpause to start rolling. I think Sony may have done that to make it more like tape recorders. Other brands of recorders (Tascam, Zoom, Sound Devices, etc.) start recording as soon as you press the REC button.
Yeah, that's one minor annoyance about the M10. I've always made sure to eyeball the lights at the start of recording, though, ever since the time I pressed the PAUSE button to release it (I thought) before the show and afterwards checked and found the yellow light still happily blinking away...
I miss the days of big chunky buttons on Walkmen where you could tell by feel if it was recording. Sort of.