The short answer is most functions related to file and input/output operations where you don’t really now how long it’ll take to do the operation.
In most cases, you’ll either know it by experience, or you’ll look it up in the documentation of the various modules/plugins. For Templater it’s not marked directly, but the examples usually give the correct indication on whether you need it or not. For DataviewJS the asynchrous functions (aka those needing await should be marked with an hourglass in the documentation.