How to focus during active video

I guess this seems silly but its not (at least to me). I've done lots of photography (with with 7D), but have only played around with video. Recently I was ask to help video some tight in shots for a Marching Band. I can't simply prefocus since the action is all over the field. And I needed to keep the camera rolling for audio sync-up purposes. Trying to look at the tiny LCD screen in the back of the camera for proper focus seem almost impossible. I noticed some of the pros used a scope attachment to the LCD to help focus. (They also had focus followers which I can understand). Any help would be appreciated.

You answered you're own questions really, it isn't easy but it's how it's done.

LCDVF + Follow Focus - or external larger LCD monitor + follow focus.

It just takes getting use to and knowing your lens well. Once you do it alot, it's not too hard. Unfortunately with DSLR's gotta do it the old way. There is Magic Lantern which has some focus options ,but they have not released for 7D yet.

Zfinder or similar...external monitor...manual focus baby!!!
I've shot for several nationally airing TV shows on 7D and 5D, 1D with great success. Takes a lot of practice to get used to it on that level. Also, until you get really comfy, dont shoot wide open apertures. the DOF is too narrow. Remember to leave your shutter at 1/30 or 1/60 if you're shooting 30fps or 1/50 if you shoot 24fps. Use ND filters to control light if there's too much or raise ISO if there's too little.

Here's a BTS pic from Guns and Gear TV which airs on NBC sports...7D with 70-200 f4 with Singh Ray thin variable ND and Zacuto ZFinder. My follow focus accidentally got left at home...didn't need it after all!

Here's a still I took with the rig... Manual focus!