Oracle SQL Developer 4: how to tell under which user I connected
You could save multiple connections to the same db but with different credentials. Give them useful names like user@thisdb and you'll be able to view that in SQL Developer at a glance in the top right corner of the worksheet.

And my favorite part: if you need to run the same SQL as multiple users or in different environments or both, you'll find that the connection name in the top right corner of the worksheet is actually a dropdown list, so it can be changed on the fly.