You really won't. It ends up being a matter of finding a trusted source. There are some larger operators (ORA especially) that sell aquacultured almost exclusively to the LFSs so ask them. As far as buying from individuals, most "frags" will be captive raised even though the mother colony may have been harvested. Watch out for some on ebay who basically buy large colonies just to chop them up and sell off. Most of those were originally wild harvested.