Piggybacking on sibhusky's comments, in the US there is absolutely no set of rules that I know of that applies across the board. I'm pretty sure it's entirely up to the individual ski area to set their own policies. At many ski areas, you could very easily get hired to be a ski instructor with no prior training as long as you can demonstrate to the management of the ski school that you have the basic skills and can communicate those skills to others. Many (most?) ski schools hold tryout camps at the beginning of the season specifically for the purpose of hiring prospective ski instructors.
Also, there's "qualified" and there's "certified". One can be highly qualified while not having any certification whatever.
The better your skiing skills going into the hiring process, the more likely you would be to pick up the movements and demos expected of you during the tryout camps. That said, my own experience is that ski school managers are FAR more interested in a prospect who has people skills like communication, empathy, and an outgoing personality than in some hotsh#t skier whose primary interest is either getting a season pass or showing everybody on the hill how great he is.
You also mention a guide job, but I'm not sure what you mean by that. In the US, "guide" generally means one of two things. The first might be someone who does complimentary "show-you-around-the-mountain" tours. That type of job is often unpaid, with the only compensation being a season pass that would allow you to free-ski during the days and hours you're not on duty. The second definition of guide would be a backcountry or out-of-bounds guide who is responsible for taking people outside area boundaries and making decisions about snow safety, routes, etc. This is a paid position but it almost always is a very difficult job to land and involves a great deal of initial and ongoing training, apprentice work, etc. At most resorts that I'm familiar with, that type of guide position is highly coveted and there's a ton of competition for the exceedingly rare openings.
Good luck with your search.