Forwarders have outputs.conf configured to send the data to indexers.
Indexers have indexes.conf and best to confirm is see the size of db files.
goto setting--> Data(indexes) and if there are good size in "Current_data" it means its an indexer.
du -h /| grep '[0-9.]+G'
if u find the index are having high disk space usage u can guess its an indexer.