How do you know if someone deleted Telegram?
Well, you won’t know if they deleted the app unless they just don’t log in for a month or a few weeks. Although you could also assume that they decided to stay away from social media and messaging for a while or something like that.
But if they delete their account, you’ll see that your chat with them now says ‘Deleted account’. Your messages should still be there, unless they decide to wipe their conversations before deleting the account.
You will know in Two ways as far as I know.
You will not see them in your contact list(if they were your contact previously.)
If you had a conversation with them before, The user would be shown as “Deleted Account” but you will still be able to see past conversations.