People who participate in certain activities that are bad for society are disqualified from testifying. Also, those who've commit certain sins whose punishment is corporal, and haven't yet repented, are disqualified from testifying. Does either disqualification apply even to cases where a single witness can testify, as on the status of food as kosher or the status of someone as dead?
As always, this question is theoretical; if the issue is practical for you, consult your rabbi.
:-(
I'd perhaps -1 this if I could. It was, as I mentioned, prompted by recent comments elsewhere, and I asked it on the spur of the moment. But it's a poor question: it lacks references (so far, and likely will, now, since they're in the answer below) and is easily answered (in the sense that if I have a source on which to base the question then I can probably easily find the answer also).