Candida species cause thrush and yeast infections. Afaik they are fairly ubiquitous in the human biome but become problematic when Candida gets out of control.

So if you can test for it chemically there could be positives even if it's not a problem. So it would be a matter of looking under a microscope at the gut microbiome and checking if theres an overgrowth. And then treatment is tricky in the gut - how do you target a single problematic species but keep the rest of the microbiome intact? And if you do knock it out, you still need to create the conditions where it doesnt just bloom again since its ubiquitous.

In the end it might be a thing where you have to follow general gut health advice and try to cultivate a healthy gut microbiome.

There are places that will do a stool analysis for you, but it's a lot of guesswork. Many quacks have also discovered that they can get people to pay a lot of money hoping for relief, and they don't hesitate to take advantage. It's a shame because there are honest people out there trying to help, but sorting the wheat from the chaff is quite the order.

My opinion is that if you're concerned, the best thing is to make sure you aren't overeating carbs (I suggest limiting carbs to 30-40g per meal and 120g per day, and avoid simple carbs) and ideally take a probiotic and/or consume fermented foods such as yogurt, kombucha, etc that naturally provide probiotics. The carb limiting is the most important. If you do nothing else, do that. Even if you don't have an issue with candida, you'll feel better overall and get healthier. Again this is just my opinion as someone who has spent years reading and experimenting on myself, but is not formally educated or trained so take with a grain of salt.

Can you narrow it down to which specific B vitamin does it? If you really wanted to undertake a project, you could do a single blind study with placebo vs real b vitamins and prove if they work (grind up real vitamins and press them back in a way they look like placebo, put them in 30 day packs, and ask a friend to label and mix them up). Then make your guess after several months.

Youtube "fungal biofilm intestine" or something like that. Take a look at the videos , its like having plastic bags inside your small intestine , it covers the intestines skin and stops it absorbing b vitamins. So you get malabsorption issues. There are many ways to test , if you mean a simple blood test where you do no work and its effortless, then no. But if you want to put in the work, you can easily test it and find out yourself, for example drinking mct c8 oil and seeing if you suddenly get very ill and need to also use the loo a lot, i.e die off herx reaction happens from the caprilyc acid , so you have fungus in your intestine. If nothing happens then you dont. Dont get caught up on what type of fungus. Anyway theres loads of tests you can do , you can try fluconazole, itraconazole , terbinafine each on their own for a week at a time and see what happens. Is there again, any die off herx reaction to any of those anti fungals. Check out the /candida sub on reddit and formulate your own plan of action though. Im not a doctor or a functional heath doctor and dont recommend doing anything I mentioned without doing your own research first. Whatever you do, you will see fungal biofilm and a lot of it in your stool , if you successfully get a die off of the fungus.