Are the cogs still on a ratcheting body? Then it's a freewheel. If not, it's a cassette.

The largest cogs of some cassettes are often held together with screws (4mm hex head or 2mm allen) or rivets.

A good way to get cogs off a freewheel is to put it on a wheel and use two chain whips to spin off the smallest cog (Hyperglide freewheels use a lockring, though).