The word you say is said aloud while the word on the shirt is in sign language. If I said the word is hello you wouldn't know what letters it corresponds to if you didn't know any of the letters.

Only you know what the word on the shirt actually is because you are trying to see if you can fool your friend. It is impossible for your friend to know what the word you said looks like in sign language if he does not know sign language!

If I told you 🤙👈👉🤙👆🫱🖐 means hello in my language and it didn't contradict any knowledge you had of the language, you would have no way of knowing if I lied or not.