I use a FLIR One on Android. I've charted internal leaks (where cold air blows in through cracks) and external (where heat escapes through eg old windows). Wait for a cold day (eg 0C), heat the house, investigate everything you can. My 1930s London house has SO many leaks, I've spent 3 years slowly fixing them. I have a talk I should give on using 12 Govee hygrometers to back-calculate moisture (absolute humidity) coherent per room, as I was charting moisture loss to trace air leaks.

If there is a thermal difference (cold outside and furnace is running, or hot and AC is running) a cheap thermal camera can find them fairly quickly.

In the US at least, where building codes favor air sealed construction, door blower tests are fairly accessible: https://www.energy.gov/energysaver/blower-door-tests

Industry standard in HVAC is to use a door blower to provide positive static pressure and then use a fogger. It works quite well since you have a direct visual indication and are not deriving loss from a secondary observation (temp/humidity).