I repeat myself runoff is worthless for soil or coco PH. If you wish to test soil PH (mostly not needed) You can do a slurry test but in order to get an accurate result you need to sample the soil in several places and depths this is really disruptive to the roots or you need a good test tool.
This is the probe I used to use connected to my Blue Lab combo meter,
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I took Harley Smiths Master Grower course and learned that the plants themselves change the PH in the biome to suit their needs. So that chasing the PH in the pot was actually stunting my plants.
The Accurate 8 or the clone of it is the least expensive fairly accurate probe I would recommend. This is the clone: