Just go through your normal watering routine. Same type of water you normally use but water until you're able to get about 2oz or so out of the bottom of the pot (that's what I use as measurement) and stick a ph pen in there.

I would just add a decent amount of perlite to your soil (20-30%) makes flushing your soil easier when you have to. Also means you get to use less MG soil :thumbsup:

let us know your soil runoff ph.