What is the Sharia-compliant standard for distinguishing between permissible and impermissible joking, and how can one avoid forbidden joking, such as scaring others, and what is the ruling on saying, "Imagine if such and such happened"?

Lying is informing about something contrary to what it is, and truthfulness is the conformity of speech to reality. There is no difference between lying in jest or seriousness. The extent to which speech is truthful or false can be measured by this definition, and the ruling does not differ whether the listener knows the speaker is lying or not. As for the saying "imagine if such and such happened," there is no harm in it because it is not reporting about reality but rather falls under the category of giving examples.