all they need to do know is see who's showing up for filming and who's not. at last check, glenn had not been seen for filming.
Well yes that would be the case. Except that they've intentionally, from my understanding anyway, mixed up the filming of episodes and parts so that even if you don't see certain people at first, or for the first part of filming, you still won't know if they're the one cause they mixed it all up.
That's my understanding anyway because they knew that people would just figure it out based on whose not there. So what they did is they filmed things out of out, some of it even way out of order, so that some people won't be there at first (even though they're actually there at the start of the season) and will be there later. So with what they're doing the point I think they're going through is that if you don't see someone there yet that doesn't mean they are killed to start the season. They might actually be there at the start, and haven't filmed yet, but that they do die later in the season.
I feel like I'm not explaining it really good here, sometimes I can't get things out on forums the way I have them in my head but anyway like I said from what I've read and what I understand who you don't see there now, and even for a while, doesn't necessarily mean who dies to start the season. The way they've chosen to film this first half of the season is supposedly so out of order that you might think someone who hasn't been there much, or at all, at the start is dead to start the season and they might not actually be dead until the midseason or another episode.
Also as far as who hasn't been seen or not, I don't know, but I read where they had everyone there to start the filming process specifically because they didn't want to give it away by who wasn't there at the start of the process.