For the past 2 or maybe 3 bitcoin tops, the best indicator was the #1 Coinbase app listing on the Apple app store. It was either #1 overall or #1 in the Finance section. You could see the chart and it basically had a huge jump and Bitcoin topped either the month before or after. IF you search Google for this topic you can see a few people who did research on the trend. It was actually pretty accurate.
Can't think of any other method. There are ways where you can see exchanges having backlogs which can hint that retail is joining and they are short of staff to approve the accounts, that was perhaps another indicator. However there is nothing public which can really pin point this data you are looking for.
It is not only the Google trends for searches on these apps, the Google trends on the words
bitcoin, crypto, cryptocoin, cryptocurrency and other words that are related and connected to the crypospace appear to have been good for monitoring retail participation in the past.
In addition to the above two signs, the sudden increase in the number of new users on trading platforms as well as the spike in trading volume in the market, especially on CEX, is also another sign. Or the unusual growth of lowcap memes or altcoins as newbies are often attracted to low cap coins with the expectation of making quick profits. Or the growth of small wallets and on-chain activity also increases significantly...in general, it needs to be combined with many factors and it will take a lot of time.
There are many websites or tools that provide this in-depth market data today, such as glassnode, nansen, messari pro, cryptoQuant or lunarCrush...but most of them are not free.