If you are really keen, you can monitor for TCP sessions on ports 8895 and 23424 between the PC Serviio is on and your devices. This can be used to infer that it is streaming.
In Windows the NetStat command will do this - look for established sessions on those ports. This method will tell you what devices are being streamed to, but not what is being streamed. It is not 100% foolproof - due to the nature of TCP sessions it may show an active session for a while after the device has been turned off or otherwise ceases playing the media (ie. it may have false positives, never though, false negatives). Haven't found this to be a big issue in the real world.
My tool here:
viewtopic.php?f=17&t=12136 does this (Windows again). It monitors these ports and determines whether a session is active, and in the case of this tool, takes action to prevent the PC sleeping.