How do I tell if a St. Augustibe lawn has Brown Patch or grubs? I can't pull up any roots when pulling on grass. Thanks

Dig up one square foot of the dead area. You only need to go an inch or so down. If you can find a dozen or MORE grubs, then you have a grub problem. If you find less than a dozen, then you have a disease problem. The turf can tolerate quite a few grubs but not a solid mat of them.