I replaced a draft inducer fan on my furnace, the two wires in the furnace that connected to the old fan are black and white. The wires from the new fan are both black. How do I know which to connect to which?
As a general rule, if the device has two wires that are indistinguishable, it won't matter. Where there's some physical reason in the construction to differentiate between "hot" and "neutral" on the wiring, the distinction is made - and sometimes it's made on things where it really doesn't matter to keep questions like this from arising.
But it's essentially the same as any non-polarized two prong plug - flip it over, plug it in, it works just the same. In this case, connect black to black and black to white and don't worry about "which black wire on the fan" if they are indistinguishable black wires - it does not matter - either can be "hot" and it will make no difference.