Hello Arnie, in most cases you should probably do a bit of research if possible. I would venture to say you may have a specific company in mind to go to work for. If not you will probably need to locate one. Which ever case applies is important because unless they have training opportunities in house you will need to be aware of their procedures and specifically what they do and how they do it. Once you have determined this you can see about going to some sort of training facility and working on learning these specific skills. Specific certifications can vary with different companies and even if you know the actual certification, many times these companies won't recognize them unless they have tested you while you are employed with them. Others may know more specifics relating to your question and will probably chime in here. Best regards, aevald

I have worked a couple of jobs in that industry (food & beverage) and at those sites, the client required a weld coupon to see if you could provide a sound weld. In all cases on thin wall stainless steel tubing (304 series) was used along with the GTAW process and an argon purge required for the inside diameter. There were several of us welders who certified on that procedure on 2"-.065-316L or 304L stainless or four, 1/2" coupons of the same wall thickness & material in the 6G position with inspection per ASME b.31.1 or 3.
Really though, you would be surprized how loosely monitered the industry as far as fabrication goes. There is alot of testing going on of the "Product" itself which one would guess covers them (and us). From what I have experienced, IF you can perform a weld (on site) which meets the industrial standard and is visibly acceptable, you can do the work. Keep in mind that  most all clients know what to look for (Quality, quantity, craftsmanship, etc.) and some are more picky than others so you'll have to be somewhat proficient to work that particular field.
Be well,

Hawk