LANE: Is it that obvious?
RORY: Only to a guy with sunglasses and a dog selling pencils.
The stereotype of a blind man selling pencils on street corners to survive was an old one even in the 19th century. It seems to be a particularly New York trope, and there is a real life tradition of blind people selling pencils on the city’s streets. There are some iconic photographs of blind men selling pencils in New York, although I haven’t seen one with a guide dog, and most of them don’t wear dark glasses.
Rory is simply saying that a blind person could see that Lane is in love.