SQL Login currently in use or not
You run a server-side trace or Extended Event session to capture login activity.

I also would not have just renamed it in production. If you are referring to the sa account itself, disable it first to verify if anything breaks; and leave that disabled for a day or up to a week. How long you leave it is based on knowledge by the application owner. If they are not confident leave it disabled for at least a month.

If you are renaming just any old login to something else for a purpose, you obviously cannot go with the disable route. In that case you just have to let your trace run for a period of time to capture the activity.

One other option you might try is to just put in a server trigger that just writes an audit of login activity. There are a few examples of this on the web.