I'm sure stewards would have to get some experience from somewhere, perhaps some of the smaller shipping lines befor the really prestigious outfits like Cunard or White Star would hire them on.
Seaman and engineers would have to present their papers which documented their previous work history and their qualifications. See the Particulars of Engagement folder on this site for more on that.
I'm not knowladgable as to how the hotel/victualling staff was picked, but assuredly, they would have been obliged to provide references much like anyone applying for a job today. How much training any of these people would have received in dealing with shipboard emergencies, I can't say, but I doubt it was very much if any at all.
Some of the people hired on didn't even work for White Star. The band for example came on board listed as second class passangers, the postal people worked for the postal systems of their respective governments, Bride and Phillips wereactually employees of the Marconi Company, and the staff of the A'la Carte Restaurant were contractors. I doubt that they were trained to do anything except their jobs.
Don't take this as gospel since I'm on some unfamilier ground in this avenue of history. I would think that Inger, Ilya and certainly Erik would know better then I do.
Michael H. Standart