This is the great black art of boxing. It's supposed to be fighting for a major organization, paid, under the pro-rules set, but everything is so subjective and varies from country to country. Some amateur bouts will allow headshots to a grounded opponent, some won't. Some pro cards have fighters way less skilled than amateur fighters from other countries. The honest answer is there is no actual way to determine one legitimate pro fighter from another until you start talking about the major organizations for each country. I've fought and been paid on various shows under pro rules but would not consider myself a pro even though, technically, my fight record says I am. I've seen many fights in elite sports fighting sessions and as far as regulation goes, although it's very common in the US, for a lot of countries boxing is still largely unregulated as they are not officially sanctioned by the local governing bodies.