How to identity contamination in mushroom cultures:

The right one at first glance looks ok, but if you look closer you can see some strange non-mycelium looking dots.

The next two pictures are close ups of the right and left. You can even see a little condensation nearby the right one, which is a sign of the heat produced by bacteria.

Final note: this Petri dish was of a clone of a mushroom, which is known to be unclean, so in this situation this was expected, and I was successfully able to transfer the clean mycelium to a new plate where it will thrive.