I think you will not be able to find this information in 9.4.2, we did not find a solution to achieve that when we were running this version.
However, you can upgrade "safely" to 9.5 MR1 (or wait until 9.5 MR4), also have a look at my post "upgrade to 9.5" for admins feedback.
The "View Reports" option is available in 9.4.2 and it's Admin rights that are needed, not necessarily NGCP. But... this report only helps identify main level data sources, no 'children' or 'clients'.
Correction (oops!).. the "Event Time" tab does show the last time a device logged. That helps unless you have some devices that aren't real chatty.
[MSSP] From a Partner: Can there be an ability to report data source inactivity on a dashboard from v10.x HTML UI. The collection rate by device query is all very well, but only goes down to Receiver/ESM/ACE level: CX is after the ability to see (and filter by) data sources.