You can cut a small branch or drill a test hole to find out if the sap is flowing. Near the end of the season the sap gets white, milky, and bitter due to yeasts. Birch sap contains the sugars glucose and fructose, whereas maple sap contains mostly sucrose. I have spent days of boiling and wound up with a bit of charcoal more than once. This year I let it go a bit too far over the cool end of my wood stove and obtained a delicious taffylike candy that tastes like molasses.
Some people use birch sap to make beer, wine or soft drinks.
The bark also yields oil of birch tar, which imparts durability to leather. The white epidermis of the bark can be separated and used as a substitute for oiled paper.