When Slide Thru is done from facing couples, the result will be some kind of box.
The dancers might end up facing into the box or facing out of the box, depending on the starting Boy-Girl arrangement.
Slide Thru can also be done by facing dancers of the same gender. In this case the result will not be a normal couple. If a Boy does a Slide Thru with another Boy, they will end up as a mini-wave holding right hands. If a Girl does a Slide Thru with another Girl, they will end up as a mini-wave holding left hands.
But you can use the "normal couple" rule to check the result any time a Boy does a Slide Thru with a Girl. If you are a Boy doing a Slide Thru with a Girl, and you don't end up as a normal couple, then one of you turned the wrong way!