Subject: What methods/apparatus would you use to monitor a source of ionisation?

For a science fiction idea. How would you go about monitoring an invisible and 
unknown source of ionisation. What is known is that the air in a room is being 
ionised, sometimes at low levels, sometimes at massively high levels, but not 
by any clearly identifiable form of radiation. The plausibility of this doesn't 
matter. What I need to know is the kind of technical methods and apparatus that 
exist to investigate what might be happening at the atomic and subatomic level.

Re: What methods/apparatus would you use to monitor a source of ionisation?