You might already know that there is some indication, but it is very easy to miss. When in multi-edit mode there are tiny lines coming out on the right from each of the selected tasks and connecting to the line going to the box where we add tags, due date, etc. If more than one task is selected, there will be more than one tiny line on the right. I totally agree with you, it would help to have something that's much more visible (those lines are very dim on my screen) and it would be good to know whether I'm in multi-edit mode BEFORE selecting a task. No matter what, it's great to have the multi-edit mode option itself. I use it a lot.