How can I distinguish between different versions of reCAPTCHA?

There currently exist two versions of reCAPTCHA, V1 and V2.

V1

reCAPTCHA V1 can take many forms, but here are the most common ones:

reCAPTCHA v1 themes

V2

The newer V2 reCAPTCHA looks like this:

animated reCAPTCHA checkbox widget