How can we precede to detect if a door is manually opened (as opposed to opened using an activation module).
There isn't a way in vanilla currently to detect if a door is manually opened. You can however logically determine it through other information. You could place two doors right after another with a gap between them, and keep in memory if the doors are "closed" or not using flup flop blocks. If someone is detected (using area blocks) between the two doors while they are supposed to be closed, they must have been manually opened.
This takes several blocks to implement, but that is as close as you can get currently. Logic is planned on being expanded in a future update.