be worth (one's) salt

To warrant respect in a certain field or area, typically because one does one's job well. Any teacher who is worth their salt would know how to deal with an unruly student like Tom. I'm confident enough to know I'm worth my salt around here. I make a lot of money for this company. If that doctor was really worth his keep, he would have caught the infection early, and we wouldn't be talking about flesh-eating bacteria right now!

worth one's salt, to be

To be worth one’s wages; a good employee. This term alludes to the practice of paying Roman soldiers with rations of salt and other valuable and essential items, whence the Latin word salarium (in turn the English salary), or “salt money.” The term was picked up by numerous nineteenth-century writers. Robert Louis Stevenson used it in Treasure Island (1883): “It was plain from every line of his body that our new hand was worth his salt.”