How can you tell if media (agar slant, agar deep, broth) that you have prepared is sterile?
The simplest way is to keep one or more slants, dishes, whatever it is that you are working with in the un-inoculated state.
Keep these controls along side the cultured samples. In a few days if you've grown colonies on your controls, it
is safe to assume that the entire batch was tainted. You need to step back and examine your preparation methods.
On the othe rhand if the controls remain clean, but you have conamination on your cultured samples, then the contamination occurred after the media was prepared and poured.