They normally take a needle suck out the fluid and check the cells on a slide under the microscope (called asperating (sp??)). Yes it is accurate, but its not the same thing as a vet suggesting to remove a lump and send it for a biopsy. If the vet is unsure or isn't confident in the slide that the cells are all just fat, they usually recommend a removal and biopsy. See dragondawg post and lizbuds link for more info about lumps.

When we first adopted Bear he had a large lump on his stomach that was diagnosed as a mast cell tumor (basically a cancerous tumor). The vets removed it and he hasn't had a reoccurence in about 3 years, but any lump makes us nervous.