Pressure leak

I have an LP gas fired boiler, all copper piping, all base board heat.

Four zone control valves with four separate t-stats.

My question is how do I find a pressure leak in the system.

The leak is so slight that it may take weeks or months for the pressure

to drop below the cold pressure limit. But then I have to add water to bring

pressure up again. Needless to say, I have learned over the years how

to drain and re-set the expansion tank. The loss of pressure only

occurs when the boiler is running frequently due to extreme cold weather.

It's a nuisance and also bad for the system to keep adding new water.

So do I need to pressurize the system with air to somehow find the leak?