I use PH strips, I get a soil sample, put in a small baby food glass jar, let it soak until settled, then use the ph strip.
do not bother with those cheap ph meters you stick in the ground, they are very inaccurate. and even if they work right when you first get them, they lose accuracy as rods get scratched and corroded, plus you cannot calibrate them ( no adjustment screw ) which requires calibration solution to do.
I've always heard that the paper is actually a lot less accurate than the meters. The advantages the paper has are: cheap, and doesn't drift. The meters are more accurate, less likely to be fooled by competing ions (but they must be maintained / calibrated...). pH testing soil is a lot more difficult than pH testing water (because of all the competing ions and various other issues).
Meters are great if you use the good ones, that can be calibrated and have good probes.
The strips i was talking about are accurate to around 0.25 pH , and not to be confused with litmus paper. so you can measure a ph of lets say 6.0, 6.25, 6.50 etc. and since your soaking a good amount of soil in water for a while, you would get a better representation of the pH your plant is growing under.
those little test kits at home depot, use just a small bit of soil and only give 10 tests per kit.
I have noticed though that the electronic soil pH meters are under 200 bucks ( digital, that can be calibrated ) so for a commercial grower like Carlos, may be ideal. for for us poor folks with just a few dozen plants. I'll use the strips, anyway these were something I already had on hand.
The digital pH meters used in liquids have really come down in price, ( thanks to all those indoor growers out there ) and these could also be used like i use the pH strips, let the soil soak in some water, and after it settles, take a measurement. I noticed some cheap 10 dollar digital "aquarium" meters made in china, and can be calibrated. of course who knows what quality these things are. best stick with pro gear if your more than just a hobbyist.
this thread has reminded me that I should be checking the pH in some of my containers which I haven't done in over a year.