"This is not universally agreed-upon, but I use ..."

This is not universally agreed-upon, but I use "$A$ decides whether or not $B$ holds" to mean "$A$ outputs $1$ if $B$ holds, and outputs $0$ otherwise".

If I said "$A$ decides if $B$ holds", I would consider that ambiguous: it might mean "$A$ outputs $1$ if $B$ holds" without the requirement on $A$'s behaviour if $B$ doesn't hold.