Easy way- look at your map. If you go to the points of interest dropdown menu you can click on each family. Each business they own has a $ next to it. if it has $ or $$ it is a business, $$$ means warehouse, $$$$ is a hub, and $$$$$ is compound.

Harder way- drive around and look for cars blocking the road with people from a rival family behind the cars. Only warehouses hubs and the compounds have the roadblocks.

As someone said before I believe, take care of the $ and $$ first, the less businesses and rackets they have the easier it is to take over hubs and wareshouses. I took over both warehouses of the Tattaglias and the Compound, by the time I got to the hub it was easier than the warehouses (for me at least).