Hi,
After upgrading Splunk Enterprise, I am no longer able to see events coming in for a certain dashboard.
How am I able to check if the forwarder that was previously sending the data is still working?
Just upgrading your Splunk installation should have no impact on data input from forwarders (7.x forwarders are compatible and supported; even 6.6 forwarders should work although it's not a supported configuration).
Check forwarder's status in Monitoring Console. Check if you have any events in _internal log coming from the forwardet host. If not, check the splunkd.log on the forwardet itself.