True, but Smart HDR is on all the time. So, it is working all the time...it controls highlights, bring out shadows, adjusting every pixel that you see. Even the reflection off your watch is controlled by Smart HDR. Or that background (star thing, poster, etc.) is analyzed and adjusted by Smart HDR.
There is no indicator of Smart HDR. Just trust it.
Exception: you can turn it off in SETTINGS...but that is not wise. Smart HDR is smarter and can do more things than your fingers can.