Answer: I think the easiest way to see if your houseplants are root bound is to unpot them and check. If the root system is extensive, the root mass will be hard and solid when you squeeze it, which means it needs to be repotted; if the roots give and the root mass seems soft when you squeeze it, there's no need to repot. Another way to check is to look for roots growing out of the drainage holes.
Hope this answers your question!
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