You can take it, but I doubt you'd pass a cert test unless you have a fair amount of hands-on welding experience. (I like to call it "spending time under the helmet.") They're not easy and they're not supposed to be.
Note that a welding cert' isn't like a doctor's license or a pilot's license. You're only certified to weld one specific kind of welding under one very specific set of conditions. For example, one particular job requires a lot of welding stainless steel with MIG, in the overhead position. So the company you work for has you take a cert' test for that. There are literally tens of thousands of possible certifications, and they only last for one year.
In a lot of cases you don't need a certificate to get a job.