If you are motivated enough. You can make an open-tube manometer with some cheap clear tubing and a glass jar or glass glass. That would read the water pressure on the other side of the water seal. It is not that hard once you get over the idea of threading a tube through your toilet trap. The tube could maybe instead be thrust through the drain and trap of the adjacent lavatory.
You can measure the air pressure at the far end of the tube. The procedure would be to run some clear tubing through a trap into the trap arm or past the weir in a toilet. Blow to remove any water in the tube.
Take the near end, and put it into a glass of water. This glass could be elevated, maybe somehow taped into the soap dish. It could be low for better positioning for video. Tape the tubing to support that to keep the end of the tubing in place. If vented, the level of the water in the tubing should be about the same as the other water in the glass. If there is vacuum in the drain air, the water level in the tubing would drop relative to the level of the water in the jar.
So figure out how to record a video of that tube in the water glass.
There are also electronic manometers that are not terribly expensive. I would not be surprised if some don't have peak / minimum pressure memories, but I have not searched.