In many retro-problems the stipulation asks who is to move
in the given position, knowing that the default rule "White
has the move" applies in positions where a retrograde analysis
establishes that any side could very well have the move.
The question can be explicit like in the following example:
The Problemist, 1930
16+13. Who mates in 1?
Here it can proved that White played last, so that Black has the move
(and gives mate).
In many problems the stipulation does not explicitly ask "who
moves?". This is very frequent with a "Mate
in 1" stipulation, which looks like a joke to the uninformed
solver, but which often really means "who mates in
1?". As a rule, in all problems with a forward stipulation, the solver
must check that the position is indeed legal, where legality includes
side-to-move assumed by the stipulation.