Blower door testing (google it!) is a great way to find out how leaky your house is, and now is a good time to do it - before it gets cold. 

A few years ago I made a cheap diy version of this, which fits over a window.  It's pretty simple to do, well in the diy category I think.  It's a car radiator fan - I picked one from a crashed sports car on ebay, and a fixed 15V 16A supply.  It works best to fit the fan blowing out of the house, so that any leakages pull air inwards, which is easiest to detect with the back of your hand.  I used it to help make our house airtight, ready for mvhr some time back.  Pulled it out again to check how things were!

If all of this goes well, you can also measure the leakyness - this involves measuring the pressure and the air flowrate.  TBH this is less useful than finding and fixing the faults in the first place.  You can make a manometer, and buy a cheap airspeed meter.

Attached is a pic of the fan in place!

I found a few places that I will fix:

Around the lofthatch handle

A few sockets still leak 

The back of our bath leaks

Toilet overflow!