You don't generally need to worry about food poisoning from beer. As long as it did ferment, there will be enough alcohol in there that nothing truly nasty can survive. There are plenty of things that can make your beer smell and taste seriously foul, but nothing that will make you sick. So as long as it smells good enough to make you want to drink it, you are safe to do so.
The only way I can imagine you could end up with serious health threatening nasties in a beer is if it didn't ferment at all (you forgot to pitch yeast, or they all died for some reason) so you just ended up with rotten sugars. It would be pretty obvious if that happened, though: you'd be able to smell it a mile off and would have way more gunk than just the odd black fleck :-)