What was the evidence that the squire had been talking too much as usual?

Captain Smollett, skipper of the good ship Hispaniola, isn't very fond of Squire Trelawney. For one thing, he hates the fact that the Squire's kept him in the dark about the true purpose of his ship's voyage, which is to search for the buried pirate treasure; he had to find it out from the crew. It's supposed to be a secret mission and yet the whole crew seems to know about it.
This indicates to Captain Smollett that Squire Trelawney's been shooting his big mouth off about the treasure map. As well as being disrespectful to the Captain personally—this is, after all, his ship—the Squire's reckless blabbering is deeply irresponsible as it could well incite this scurvy crew to mutiny.
Smollett's already none too pleased with the crew he's been forced to take with him on the voyage; he rightly sees them as a bunch of greedy, dishonest cutthroats. But now that the big blabbermouth Squire Trelawney has gone and told the men on board about the treasure map, he's made things even worse for Smollett by effectively dangling temptation in front of their faces, greatly increasing the chances of a full-scale mutiny. If Captain Smollett ends up walking the plank, he'll know who's to blame.