What is Smaug’s clever question (the catch)?

From the text:

Yes, it is rare and wonderful, indeed," said Smaug absurdly pleased. He did not know that the hobbit had already caught a glimpse of his peculiar under-covering on his previous visit, and was itching for a closer view for reasons of his own. The dragon rolled over. "Look!" he said. "What do you say to that?"

"Dazzlingly marvellous! Perfect! Flawless! Staggering!" exclaimed Bilbo aloud, but what he thought inside was: "Old fool! Why there is a large patch in the hollow of his left breast as bare as a snail out of its shell!"

After he had seen that Mr. Baggins' one idea was to get away. "Well, I really must not detain Your Magnificence any longer," he said, "or keep you from much needed rest. Ponies take some catching, I believe, after a long start. And so do burglars," he added as a parting shot, as he darted back and fled up the tunnel.

It was an unfortunate remark, for the dragon spouted terrific flames after him, and fast though he sped up the slope, he had not gone nearly far enough to be comfortable before the ghastly head of Smaug was thrust against the opening behind. Luckily the whole head and jaws could not squeeze in, but the nostrils sent forth fire and vapour to pursue him, and he was nearly overcome, and stumbled blindly on in great pain and fear. He had been feeling rather pleased with the cleverness of his conversation with Smaug, but his mistake at the end shook him into better sense.

"Never laugh at live dragons, Bilbo you fool!" he said to himself