Your new employer will have to qualify you to their welding procedures before you can perform any production welds. The question is, "How do you show them that you have the skills required to pass a qualification test?" You can spend the money and go to a commercial laboratory and take a test with them, but you will be charged for the test (pass or fail) and it isn't cheap. Then again, some people spend a tidy sum to hire people to write their resumes. Nothing worth doing is cheap or easy.
The next question is what test do you take? The answer to take depends on the type of work your prospective employer does. You can't go wrong if you take a pipe test on stainless steel using the GTAW process. If you are up to it, take the test on pipe in the 6G position, open root, i.e., no backing ring. One inch heavy wall would be the option I recommend. That test is applicable to aerospace, piping, and any other welding that requires GTAW. The cost of the test can vary widely depending on the location of the testing facility. Here in the Northeast, the cost of testing is higher than it is in the Southwest (based on some of the postings I've seen here). Bear in mind that many shops expect the welder to have the ability to use more than one welding process and base the welder's pay on their versatility and experience.
The number of different qualification tests given by industry is nearly infinite. You could take tests on aluminum, titanium, nickel, etc., but the chances are it will not be the alloy welded by the prospective employer. The intelligent employer recognizes it is nearly impossible for prospective employees to be tested on the exact base metal, product form, process, and position as they are using in production. However, an applicant that has recently passed a standard test has demonstrated the basic skills needed to weld regardless of the specifics of their production requirements. The prospective employer should be interested in whether the applicant is "trainable" and whether they are a good match for the type of work they specialize in.
Best regards - Al