As usually that depends on what you have on those UF / HFs. How many logs, what are volumes of events and in HF have you running also some other inputs like DBX etc.
You should use your normal monitoring tools to see what happenings on UFs. With HF you could also use MC (monitoring console) and add those HF's as indexer to it. Then just create own custom group for those and separate for real indexers and then you could follow up those with MC. After that you could also see some other splunk's internal metrics like HEC statistic, some queues, pipelines etc. on those.