"A food and symptom journal can be helpful for trying to connect the dots."This is a good idea, I will try this once I start cutting out foods again.
You should start the journal now. You need to establish a baseline to compare things to. Start it well before you start making changes and don't make more than one change per week. Reactions to changes can be delayed and it's pretty complicated stuff.
How does one go about testing this?
Sorry, I don't know. I never figured out the secret handshake or whatever to get the medical system to give me the right test to actually identify my infection.
I looked up my symptoms online, drew my own conclusions and I've gradually gotten healthier, though I get treated extremely abusively for trying to talk about it online so I got nothing more for you than the two tidbits above.
Best of luck.