I love my BL soil Ph Pen. poke a hole, insert pen, read Ph, done. But checking the runoff of the water is effective tool as well. What people don't understand is that you can not water a higher Ph than your soil and expect it to come up, at least from my experience. The only way to improve the Ph was with dolomite lime or a similar product. something needs to buffer that shit!
@PlantsAreNeat having a root bound container is not the end of the world either. I actually prefer my plant to fill in the container heavily by about week 5 of flower. The benefit to having a pot full of roots is that you know the whole container is the root zone. Some people don't understand the root zone and knowing where you need to put the water and nutes. when you have a container that is too big and water it heavily, the plant cant transpire fast enough (or dry the pot fast enough), and what happens is it eats all the food by the roots (root zone) and then starves because you can't water and put more food into SOLUTION because the soil is still wet. Not to mention when one oversizes a plants container it's wasting water, soil, nutrients, resources which cost money. so if it takes 4-5 days to dry, thats 2-3 days too long. The more often you can water (by having the pot going dry faster) the more CEC takes place. CEC = cation exchange capacity, this is how plants take in nutrients from the growing medium. If you don't know what this is look it up, and if you need help having it explained shoot me a message.
So before you transplant wait for the pot to dry and water it, check the Ph before you water, and check the runoff. if it comes out 6.1 or less you need to get some dolomite lime, and topdress roughly 1.5 tablespoons into the top 2 inches of soil and then water it in your next water with Ph between 5.8 and 6.2. the lime will float your Ph to a stable 6.5 until its dissolved after so much time and watering. You dont even need to test, you can just add the lime anyway, it wont hurt a thing. actually if you buy a quality lime, it is derived of Calcium and Magnesium which is great for your plant. you may not even need to transplant if you can water daily or every other day when the pot starts going dry faster. just remember you have to use less nutrients than the bottle suggest, because those feeding suggestions are just that: suggestions. They don't know how often you water.
FWIW, when i take clones and they root, i put them in 3/4 gal containers for roughly 3-4 weeks which allows them to get a very nice healthy root ball. After that, i transplant them to their final home, a 4 gallon pot, which is just the right amount of space to veg for ~2 weeks so they can acclimate and get roots going, and then flip into flower worry free. by week 5 the containers are going dry every other day and i can feed my plant less nutrients but more often. this increases CEC big time. which i promise will lead to higher yeilds. I have many friends with 1000watt hps that can't put the numbers out that i do with my 600watt. 1000s are overrated, energy sucking, $ sucking shit. The sun hits the earth at 700watts per cubic meter, so why use a 1000 watts? sorry im just ranting now. let us know what happens with the runoff Ph when you get to it.