You can tell you are meeting their needs if they grow well, look healthy and produce well. What they require is harder to define because it will vary depending on your soil and your climate (especially the amount of rainfall, since heavy rainfall can leach away nutrients). I generally just do the soil improvement before planting and then trust that the plant roots will stretch out as they grow and will seek out, find and absorb whatever nutrients they need. I always intend to go back and side-dress medium feeders and heavy feeders periodically and then I get busy and seldom actually do it, and my plants still produce just fine. Keep in mind that I started out with red clay that was poor in drainage but very high in nutrients to begin with. Also, I generally mulch with a thin layer (1/2") of compost applied to the top of the soil with chopped/shredded leaves or grass clippings on top of that. I add more grass clippings every time we mow. Every bed will not necessarily get grass clippings added weekly, but all beds get them added at least a couple of times a month. Those grass clippings don't just sit there and serve as mulch. They break down and feed the soil continuously and seem to provide all that my plants need. (Maybe this is why I can get away without top dressing.)

Because I largely rely on compost-improved soil and Espoma fertilizer applied pre-planting, I don't know how to answer your 5-10-10 or 5-10-15 question since I don't use those fertilizers. Just follow the label directions on whatever fertilizer you buy. If you have sandy soil, leaching of nutrients may be a problem until you get the percentage of organic matter in the soil a bit higher and supplemental feeding every month or two may be necessary (and even more necessary if a hurricane blows through and leaches out all your soil nutrients).

The linked chart from UGA shows how to calculate the amount of N-P-K needed and apply those nutrients.

Fertilization Table