Google/Facebook logins on third-party apps

How do I know I'm secure when an app asks for a Facebook or Google login? For example, a browser will redirect me to facebook.com or google.com to give permission to a third-party (I can see the URL up top). But on a phone, there's no URL to see. It just shows a screen that looks like Google/Facebook log in page, but I can't know for certain without the URL indicated (it could be phishing for all I know).