When do you know when to use the disk and washer method?

Disk and washer methods are methods for solving the volume formed by revolving a function(s) about an axis. The disk method is used when the volume enclosed is solid (like if you have x^2 that you rotate about the y or x axis). The washer method is used when the enclosed volume is not solid from all the way to the axis (like if you had the region between x^2 and x rotated around the x or y axis)