Heat sensitive patches designed to check autoclave temperature.
Autoclave bags have spots of indicator tape on them, or the tape can be purchased separately. When run through an autoclave the spots change color in response to temperature - Note that these spots only indicate that the autoclave has reached the proper temperature. They do not tell you if the proper temperature was reached for long enough, or if the pressure was correct.
If the spots don't change, it is an immediate warning sign, but spots changing are under no circumstances proof that your autoclave is functioning fine. A regular program of spore testing as well as knowledge of how to properly use the autoclave is essential.