Check out the solution in the video TSQL: Quickly Obtain Log Space Used (DBCC SQLPERF). Prevention beats cure, but this is a great command for those situations we haven't had time to look at and need to get information quickly. Also, we see how we can use it to clear metrics, if we need to in our situation. This won't always apply.
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While getting the log space after the fact may be reactive, I understand that there are some environments we're completely new to or run hundreds of servers and may not have had the time to do a quick analysis. If you can, evaluate how logs are set and used before reacting to a situation.