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How can someone know if messages were deleted from my phone other than myself
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If you are referring to the Apple Messages app then only you would know if messages were deleted from that app. NOTE: If you are sharing your Apple ID with someone else then that person will also know.
There are several third party apps (example: WhatsApp) where you can delete messages and the other person will be notified as well unless you specifically choose to not allow that.
Axel F.