How would I test if my custom dimension exists?

I’m making a datapack that relies on a custom dimension. If the dimension hasn’t loaded in, it will break things. When you add the datapack to a pre-existing world, then the dimension won’t load until you relog (singleplayer) or restart the server (multiplayer). I’m trying to setup a function that repeats every tick (I know how to do that already) that will check if the dimension has loaded, and if it hasn’t, send a message to restart the server/world. I’m trying to do this with /execute if, but that doesn’t have a way to check dimensions (same with /execute unless). Another idea would be /execute in, but that can’t return true or false.