We’ve had a question about this in our house which is 100+ years old too. I bought a £5 hygrometer off Amazon, this measurement will tell you the humidity level in the room or wherever you place it. Anything over 65-70% will start to see damp, mould, respiratory conditions and the rest, but it helped me to know exactly what the situation is. Our bathroom needed better ventilation for example as it’s a wet room, same for kitchen. It’s pretty obvious really, you can see moisture on windows but it’s the moisture you can’t see which is what you’re measuring.

Indoor plants love moisture, they can take care of some of the problem. Our kitchen window plant is out of control, very happy plant.

Water in brickwork is really bad. It can start at the base of the brick wall and rise through hydraulic action, or if there’s a loose gutter at the top it only takes a hole under 1mm to be a constant drip which never allows the brick to dry. Also could be the brick pointing, if it’s old pointing cement it might have turned to sand and blown away, so the job of protecting the brick is lost and water could be going inside rather than down the face. This is an easy job but not cheap so better to find a brickie for an assessment. Likewise cracks in render allow water in but not out. You can run round with a sealant tube and it might solve it.