By law the slow-release percentage must be listed and usually the source of the slow release properties. But the label may exaggerate or might try to fool you by saying, "contains 15 percent slow-release nitrogen". Or it may say, "Contains 50 percent sulfur-coated urea".
15 percent slow-release does not mean on top of the 30 percent already specified--rather you have 15 percent slow release and 15 percent fast release; total 30.
Some professionals want a low percentage of chloride, (want no muriate of potash).
And 50 percent sulfur-coated urea is not exactly 50 percent slow release--some of the "coated " fert is not slow release due to normally a percentage of the granules are cracked open. Read the label carefully and be a bit skeptical. I don't know if there is such a thing as slow release phosphate. Slow release potash is rare.