Recently a friend gave me the credentials to his Google account (GMail and password), so I could check his email while he was on a trip (with no cell signal). He is a trusted friend and not especially tech savvy, so I don't suspect malicious intent. For the record, I thought this was a bad idea, but he insisted.
I was able to log into his GMail account and check his email as well as every other app that used Google for authentication. However, I did not give him my credentials, I only put his account on my device.
At this time I have access to all his information, but I'm worried he has access to mine as well. Is there any way I can check? Is this even possible without him successfully logging in as me once (keep in mind he doesn't have my password).