How do you find out if your Featherweight is 221 or 222 or whatever numbers there are?

A 222 is a free arm.

Usually there is a model number on the machine. However, it's very easy to tell a 221 from a 222 simply by looking at it. The 222 is very rare and has a removable "table" to turn the machine into a free-arm. The 221 does not have a removable table but instead has a permanent hinged table to the left of the needle.

Most are 221 since the 222 is so rare.

Go online and there are a lot of picture for these machines.

The older Featherweights do not have the model number on them. I don't think Singer started putting a model number on them until the late 1950's. My 1950 Featherweight has no model number but my 1958 has 221 on it.