How would you construct a so called not-joke in french, where you say the "not" in the end to create irony.
E.g.: The sky is red, not!
I suggest "ou pas" :
"Le ciel est rouge, ou pas !"
Ou pas is idiomatic but leaves some uncertainty not present in the English "Not!"
An alternative expression can be Eh bien non ! often simplified in spoken French to Eh ben non ! or Et ben non !
Le ciel est rouge… eh ben non !