I'm investigating a blocking issue that I believe is related to implicit transactions not being committed or rolled back.
How can I identify connections that have turned the implicit transactions setting on?
Well to answer your specific question, you should be able to see that in the existing connections and audit:login events in SQL Server Profiler/Tracing.
I would actually start by trying to investigate who the blockers are and then dig in, however. You may have better luck in actually tracking down the blockers if they actively block and you see the blocking in action.
You should be able to do that by either querying the dynamic management view - sys.dm_os_waiting_tasks (described here) or installing and using Adam Machanic's sp_whoisactive (a procedure I recommend you install anyway)