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They will know probably after they fix it. If you rooted it and used a different firmware on the device they can recognize the changes from it's previous memory.

If you can connect to the device from a PC then they will most likely be able to tell. It might not matter though because they might not look for that and unless they intentionally look to see if its rooted they will not know. Also like @Poldie said they might not legally be able to do anything about it depending on where you live.