That's about all I do do, look for any sort of Movement from either the Bird or the grass that it's in. Sometimes it's just a brief 1/2 second where the grass might move up slightly, then you have to decide whether it was from the bird or the wind. I wouldn't worry to much about getting/marking them wrong, I get some wrong myself.
Their going to be going to a different Viewing Interface pretty soon, I've already run a lot of Video's on it already trying to get used to it. So all the Video's in the Old Interface we're just trying to blast thru them as fast as we can, if there's any doubt at all in my mind whether the Bird is there or not I just mark Unsure for Bird Present & Absent, everybody else is doing the same. The more you watch the more you catch on to the birds movements & how to spot them if their there ...