Is there any definitive statement within the Predator movies, books, comics or games concerning whether Predators can hide from each other? Do any of their helmet's enhanced vision systems negate the effect of their stealth armor?
Depending from the game it was either a separate vision mode or cloaked Predator was visible in thermal:
In the Aliens versus Predator video games, the Predator has four vision modes, regular vision (to fit in with the first-person shooter genre), thermal (spotting regular beings like humans), electric (gives a red background, used spotting Xenomorph's by their white outline), and Predator vision (gives a green background, used for spotting other Predators and technological objects, such as weapons and armor, shown in a white outline). [...]
However, in Aliens vs. Predator, there are only three Predator vision modes: Normal, Thermal/Heat, and Xenomorph. Normal mode is self-explanatory. Thermal vision is used to detect human and synthetics (and Predators — cloaked and uncloaked — in death matches). Source