Signing up with Google + Terms of Use checkbox

I'm stumped: How do you nicely solve the requirement of explicitly checking the "I accept the Terms of Use" checkbox when using single-click sign up with Google / Facebook / Twitter?

Before we had "by signing up you accept our Terms of Service", but lawyers told us that's not good enough and the users have to explicitly agree to terms of use for us to have legal protection.

Looking at it from the user flow perspective, it feels really weird to have (for example) buttons disabled until someone accepts Terms of Service, showing it after signup feels even weirder.

Any of you people seen this solved smoothly?