Rheumatoid Arthritis Support Group
Rheumatoid arthritis is a chronic, inflammatory, multisystem, autoimmune disorder. It is a disabling and painful condition which can lead to substantial loss of mobility due to pain and joint destruction. The disease is also systemic in that it often also affects many extra-articular tissues throughout the body including the skin, blood vessels, heart, lungs, and...

Diet makes no difference to me at all! I've tried various diets, and nothing helped but strong meds. Or is he testing for allergies? I have loads of allergies, but they don't connect to my RA at all.
You need a hypothesis (for instance, item X might case inflammation) and a way to test that theory, and a way to assess the results Ideally, you'd use controls and blind tests and all sorts of things that become impractical when you're subject is your own self.
Another challenge is to find a baseline diet that would not cause inflammation, so that you've have a starting point. But at the very least, you (we, because we're all in the same boat here) can change only one factor at a time, and give it enough time to work, and figure out how to assess the results in an objective way if possible (measuring the circumference of your ankles, for instance, if they're prone to swelling, or comparing before-and-after photographs of red joints, as opposed to deciding if you 'feel better' on particular day.)
So, you might start with a safe diet that causes you no problems, and add seafood as your X factor for three weeks, and see if you have skin or gastro issues or whatever. I don't know if there's a link between RA and diet, but it stands to reason that anything that knocks us out of balance in non-RA ways, even if it's just an occasional upset stomach or bloating due to dietary choices, can't be a positive thing. So why not get as much information as you can about your own personal self? Good luck to you.
However, if you're trying to determine if someone is sensitive to food, an elimination diet would probably be a good way to go. Maybe there's a group that focuses on food allergies?
There's been good suggestions already. I would only add that you need to do them for a good 4-6 weeks to really know and pay attention to when you add a food type back in to see if it's making you feel worse.
This is not as onerous as it sounds and I think worth a try. Good luck!