The only way you can ensure you are secure is if you use a trusted mobile application, such as Chrome, and you see the padlock and HTTPS at the start of the address bar.
Note that here you are trusting both Chrome and the domain of the website that is HTTPS.
If you're using a third party application that integrates a browser, you are trusting that application. If you don't trust that application, you cannot trust anything displayed to you within that application. Therefore, you should only enter credentials through applications that you trust, in this case Chrome.
Note that even if you trust an app, you need to beware that the app loaded could be subject to scheme hijacking and it might not be in fact the app you expected to load. Always load up the application manually before entering any credentials.