How to tell the difference
well, 'beaded' styrofoam looks like the white stuff you see making up coolers, paper cups, cushioning in shipping boxes, and the like. Tends to make a squeeking noise badly affects people with sensitive hearing.
The Extruded foam is used as insulation in walls, and has a much finer 'grain' than the beaded stuff.
a couple pics
Here is the white, 'beaded' styrofoam
extruded styrofoam is the pink stuff you've seen all along in my layout. It can also come in blue and green
Its comes in 4' x 8' sheets, from 1/2" to 2" thick