Using the static exporter option from,
https://github.com/lukemonahan/splunk_modinput_prometheus
Where can the status of the exporter and whether data is being polled checked ?
A search similar to this should give you log output from the input. The modular input will dump an error to splunkd.log if there is a problem with communicating with a Prometheus HTTP endpoint:
index=_internal host=<polling host> sourcetype=splunkd ExecProcessor prometheus