I have a Shiraz that had completed fermentation and clarification and was being stored in a carboy until I had time to bottle. Apparently, the air lock was knocked off and it was sitting open (with a dark towel over the top) for up to 7 days (the last time I checked it). Is there any way to check for infection or is it a lost cause?
It's very unlikely that you've got an infection. Wine traditionally has been brewed in open vats for hundreds of years. The risk of infection is much less than it would be if you brewing beer. Vigorous yeast, high alcohol and low pH keep most things at bay.
You may have got a little oxidation, but don't give up on the batch because of that. In the worst case bulk store the wine and use it for blending.
If you're really worried about an infection that will develop in storage you can pasteurise the wine in a dishwasher.
As for testing you can test microscopically, but taste is the best indicator.
The only general reason I see to worry is if the yeast didn't get a good head start before the airlock came off - unlikely to be a problem if it was CO2 that blew the airlock off. Even then you might be worrying about nothing. The other thing I can think of to worry about is an obvious proximity of some must/wort-loving micro-critter. I'm pretty sure I've had infections traceable to the litterbox when I brewed indoors in a small apartment. But, again, that's hardly a guarantee of a problem, just a possible source for one.
Concur with Kevin Sharp. I'm not even sure I'd worry about the oxidation because CO2 is a heavy gas that will tend to just sit on top of the must if left undisturbed. I would do nothing and just see it through. Then do a taste test when you normally would.