At least in the US, blower door testing:

https://en.m.wikipedia.org/wiki/Blower_door

I've had this done before, very, very informative. Those machines are like $3K to buy, you can hire someone to come out and do it, but this seems like a perfect DIY project.

An other very informative thing to get done is thermal imaging of the walls (from inside and outside both). Very good at uncovering damp spots and water infiltration too.