I second what Sue said, in the sense that our individual needs will vary. Keep in mind that over-supplementing can be just as dangerous as under-supplementing, especially if you start playing around with things like iron, potassium, etc. Worst case scenario with over-supplementation is that you can give yourself an overdose; least case is that you're paying money for stuff that is just running straight through your system. I do currently take several supplements (iron, multivit, vit D/calcium, B12 and omega-3s) but ALL of these have been recommended to me at some point or another either by my doctor or dietician. I admit that some days I probably don't drink as much fluid as I should, but I try to urination as a guide (not to be too graphic *LOL* but obviously if I am peeing too much or too little, I will adjust accordingly). My dietician did tell me that thirst is not really a good indicator for fluid intake, because usually you are already dehydrated by the time you feel thirsty. So something to consider.
But, apart from a multivit and calcium, which most of us should be taking anyway, bloodwork is definitely a good start to see what you might be deficient in. In general, studies have shown that IBD patients are often deficient in potassium, iron, B12, magnesium, and vit D, but it doesn't mean that all of us will be deficient or that we are deficient in all of those things simultaneously. And there can be other mineral deficiencies as well, not all of which are tested for on routine blood panels. That's why it's a good idea to talk to your doctor about what you may need or what you may not be fully absorbing and go from there.