You can use the following query to find Session Id of a Concurrent Request:
SELECT ses.sid AS session_id, ses.serial#, fcr.request_id
FROM v$session ses, v$process pro, fnd_concurrent_requests fcr
WHERE ses.paddr = pro.addr
AND pro.spid = fcr.oracle_process_id
AND fcr.request_id = :concurrentRequestId;
:concurrentRequestId is painted red area in the image below.