How do you tell what kind of lump in your breast might be cancerous from the ones that are not?

"Different": Feeling for a lump in an organ that by it's very nature is lumpy-bumpy can be very confusing; usually, it's not a question of whether you can find a lump as much as how many. However, cancers often feel distinctly different than normal breast tissue: rock-hard (like it is made of concrete rather than rubber), 3-dimensionally round (rather than pancake- or cigar-shaped), and fixed w/in the breast.