The timeline automatically sets the direction property on a provided object given the following:
Timeline.SetPropertySetObject(UObject); // Object to set the property on
Timeline.SetDirectionPropertyName(FName); // Name of the UPROPERTY
So a final setup would look somethnig like this:
MyActor.h
FTimeline Timeline;
UPROPERTY()
TEnumAsByte<ETimelineDirection::Type> TimelineDirection;
MyActor.cpp
Timeline.SetPropertySetObject(this);
Timeline.SetDirectionPropertyName(FName("TimelineDirection"));
Then you can use the following variable and it will tell you the direction
TimelineDirection == ETimelineDirection::Forward
or
TimelineDirection == ETimelineDirection::Backward
good luck