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 any case, it appears that the pumps will continue to have much resistance hehehe. These good news from the Donald is not enough to give everyone the pumps similar to 2017 and 2021.