Infections are pretty obvious when they get to be bad enough to cause health problems. Anything you're likely to get sick from will smell and taste like something to be spat out. Mild infections might not have a visual cue, but are often accompanied by volatiles (often vinegar-like or acetone-like smells). Larger infections usually have some kind of polymeric goo surrounding the bugs, such as a pellicle (on the surface, a whitish bubbly layer) or ropes (into the liquid, look like ropey snot), or even something similar to a mother (blob on the bottom, you can see these in unfiltered, unpasteurized vinegars).
The first time you make mead, it will smell and taste funny at first. Often the "green mead" smells and flavors are confused for infections by beginners, as they are not standard aromas and tastes found in finished wines. However, these do not have the characteristics above (vinegar or acetone smells, snot-like goo) so don't freak out right away.
Organic acids have a smell, and these can be quite strong at first. The new ethanol also tends to taste solventy and harsh right after fermentation, but it will mellow with time.
If you want to try the wild yeast idea, use some yeast from a sourdough starter that you've had success consuming in the past. This eliminates the chance involved in capturing something from the air. You will still get a mixed culture, but at least you know where it came from. I would also suggest doing a few small batches first, serially pitching the yeast from one into the next. This helps kill off bacteria that is not alcohol-tolerant, and you should get a cleaner result than if you made your batch straight from the bread starter. Using musts with OG around 1.040-50 for these "weed-out" batches would be my suggestion.
I have successfully captured wild yeast, as have several others here, so it can be done. My batch details are
here. Perhaps some good information can be gleaned, there are also links there to other experiments.