I mean, we all have our tell-tale signs, right? Personally, I keep an eye on Google searches with keywords related to Bitcoin to see if retail interest is picking up. And I know people are really getting interested when my gardener starts talking about Bitcoin!
This one right here is the biggest one for me. When random people all of a sudden start talking about investing in Bitcoin (or often in other crypto, because, you know, Bitcoin is too expensive), then you know we either reached the peak or are about to.
Another metric is the growing popularity of Bitcoin trading apps on Google Play or in Apple Appstore. I started a similar topic a long time ago and somebody pointed out there was a correlation between the Coinbase app being ranked 1 in Appstore and Bitcoin's price reaching the peak.