take the rear wheel off, clean up the sprocket end as much as practical, and take a good SHARP well focused and reasonably well lit photograph, and post it here, and we'll guess for you.

sheldon's photo of a cassette:



and freewheel:




now looking at it like that, if you backspin the cogs when the wheel is stationary, does the spline move with the cassette or stay fixed? if it moves, its a cassette. if it stays fixed with the wheel, its a freewheel

there's NOTHING on that page of Sheldon's about a bulge.