I feel like I am parroting myself a bit, but I have some water test equipment (a set of three jars of test strips) that were under $100 AUD for the lot. Two of them have nickel litmus pads, so I know it’s easy to test water. As far as food goes, I don’t know what the science of the litmus is.. it’s made for testing water. When you suddenly put food on them, maybe they give false readings on the nickel. You sort of need to be a chemist, or know a bit of biochemistry to use things out of the primary designed-for use case.
Hmm, still, those strips get really cheap when you buy them in smaller quantities. I imagine you could formulate some self-tests, but the big thing is to get something that is a known nickel containing substance, and know how to dilute it down to trigger the different values the colour indicates.
So, you’d test your ‘home made diluted nickel calibration fluid’ and confirm the values you create are detected accurately by the strip.
Then, you’d find some foods that have no nickel, and test mixing the foods with a quantity of the calibrated fluid.
Then you’d be set to test a wide variety of foods you suspect might have nickel.
But really, it’s very complicated. I mean, if you’re talking about nickel scrapings from stainless steel implements in a stainless steel pan, or a cast iron pan, or on the rough worn portions of a porceain bowl, like if you’re eating cerial, the scrapings are metallic, and if you consume them, it’s metallic nickel, not in a colloid or actually in solution. In such case, I think you could have nickel consumed but not trigger any test strips.
Testing isn’t easy, I wish I knew something about it other than guesses and assumptions.