“Check if you did it or not” isn’t wrong exactly, but it sounds a little clunky, and it has a slightly odd vibe. The “or not” at the end is redundant because “if” already implies the two possibilities. You’d normally only keep “or not” when you want to emphasize the alternatives, like “I don’t care if you did it or not.” But in a plain instruction, “Check if you did it” is cleaner and gets the job done.

The bigger issue is that “check if you did it” sounds like you’re asking someone to verify their own past action, which can feel a bit weird depending on who’s talking. Like, if your boss says it to you, fine, it means “go back and confirm whether you completed the task.” But if you say it to yourself, it’s more natural to say “Let me check if I did it” or “Did I actually do that?” So context really changes everything here.

If you mean “verify whether the task was completed” in a work or school setting, you could say “Make sure you’ve done it,” “Double-check that it’s done,” or “Confirm whether you completed it.” Those all sound natural and don’t have that redundant “or not” tail.

If you’re talking to yourself while looking something up, “Let me see if I did that” or “Did I do that or not?” works fine — here the “or not” actually feels okay because you’re genuinely weighing two options in your head.

And if you’re being playful or sarcastic, like you suspect someone’s lying about doing something, you might say “Check if you actually did it” with emphasis on “actually,” or “Go on, check whether you really did it.” That adds a little bite.

If you’re writing something formal, like a report or instructions, “Verify completion” or “Confirm whether the action was performed” is the way to go.

https://notadictionary.com/is-the-way-to-go-how-the-structure-works/