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But does someone have to declare that they are all in?
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See page 9, Items #13 and #14 from
Robert's Rules of Poker.
No, the act of simply moving all of your chips into the middle is a legal action. You are not required to verbally announce an all-in or the amount you are betting. This should however be the dealer's responsibility, but at home games, you can't expect this too often.
FWIW, if he is concealing his chips with his arms on the table, you need to ask "how much do you have left". It sounds as if you weren't aware of your surroundings if he can move all of his chips into the center and you didn't pick up on this. Before looking at your own cards, wait until the action makes it to you to look at the cards and decide; that way you are free to watch the others for tells and be aware if another has gone all-in.