In my testing I haven't found organic produce tests higher BRIX than conventional produce.
The organic stuff does have the advantage of not having any pesticide residues, but you can grow low BRIX produce organically. I personally grew some pretty terrible strawberries this last time around ;).
Every time I try to come up with some general rule as a substitute for testing, it never holds up.
I wish it were easier but given the state of our food supply, there seems to be no alternative to learning how to grow high BRIX food and testing vigorously to produce that you buy.