Sometimes you really need to know about your dirt. Maybe your plants aren’t happy, or maybe you feel they could be happier. There’s only one answer: spend the money and test your soil. Typically, you get an agronomic (or garden) soil analysis that tells you what’s in your soil, if there’s not enough of something they need or too much of something that’s going to negatively impact their growth.
Luckily, there are lots of services that can tell you all about your dirt, and thanks to the University of California, you can likely get everything you want by contacting a lab from their list (pdf file).
Typically, all you’ll need is a soils test, although if you’ve got the money, they’ve likely got a test for whatever you want to know: toxic chemicals, pesticide residue, pathogens, nematodes… In any case, it’s likely best to start with a basic soils test and work backward from there.