If it looks like a duck and walks etc. like a duck
If it looks like a duck and walks etc. like a duck
If it looks like a duck and walks/quack/flies etc. like a duck, it is a duck.
(humorous)
used to say that something is probably exactly what it seems to be and we should trust our judgment about it They're calling it a clinic, not a prison, but if it looks like a duck and swims like a duck, then it is a duck, I think.