You can look at Google trend and some indicators for Bitcoin bubble that bases on Google trend and some stats from social media platforms.
https://trends.google.com/trends/explore?date=all&q=bitcoin&hl=en
https://www.coinglass.com/pro/i/bitcoinBubble
https://www.theblock.co/data/alternative-crypto-metrics/web-traffic

You can look at Bitcoin heatmap that is given by Glassnode in their report like this one
https://insights.glassnode.com/the-week-onchain-week-18-2024/

You can simply see what's happening around you. If many people around you start to talk too much about Bitcoin, this market while just previous months, they did not care about it, it's good signal that retail investors are coming.

Google trends + exchanges volume + etf volume, pretty easy formula for raw estimate

Retail investor are the most confused souls as they are the first one to do FOMOing or even panic selling. It's not new we have been witnessing this for years now and I think this is something which benefits whales more than anyone as they try to shape market behaviour to their benefit. But I don't think any tenured user will not be FOMOing as they accumulate it during the bear as well and do DCA throughout the year as they don't overbuy or panic sell

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.

Sorry mate, but are new comers not welcomed at BTC Talk? I am generally asking because I want to know what others are checking.

When retail buys, I start to sell.

I think historically Google Trends has been an excellent way to see with data whether or not the average person is looking into Bitcoin.  Currently it's almost like Bitcoin is still at the cycle lows the way it's being ignored by the average person.  I think a lot of people got burned in 2021 and won't give Bitcoin a second look as a result, but there are still plenty of young folks who are stacking and being introduced to crypto at younger and younger ages.  I think we probably have a year or slightly longer before Google Trends maxes out in it's Bitcoin searches, but when it happens I don't think you'll need to go searching for ways to see it.

I don’t think I can share a very accurate one, all is just simply observations anyway. Everything is speculation so we can not expect a 100% accuracy.

However, I think social media can be a good indicator of what is trending or what’s not. So, by looking at it we can determine where the retail investors are going for as of the moment. Of course, market performance is also a good indicator

There are some reasons for Bitcoin FOMO. So you have to stay updated about upcoming Bitcoin events. For me, I do follow some free Telegram channels where I can get early news. They do research and share it on their channels. So I don't need to search on Google or any other specific websites. FOMO always comes from positive news. So it's very important to gain knowledge about upcoming events. You need to find such good channels from which you can get the latest news quickly. Otherwise, every time you would need to use Google,. 

Just wanted to point out this is a terrible indicator. I see people make this sort of claim all the time. A single person talking about Bitcoin means nothing. People all the time are finding out about Bitcoin and getting into it. People always make this claim: as soon as a person I know in real life mentions Bitcoin I know the market has peaked and its time to sell. lol. That is horrendously anecdotal and means nothing. A gardener, or taxi driver, or dentist, or whoever, is just as likely to get into Bitcoin as anyone else. Having a single person mention Bitcoin to you in real life says nothing about the state of the market.

If it was like all sorts of people you know from different walks of life are suddenly all talking about Bitcoin, then that would likely signal a retail frenzy is going on and the top could be coming in days/weeks/months. That's very different than "a single person I know mentioned Bitcoin" lol. I always cringe when I hear this sort of claim.
End of rant.

Aside from google trends, when you see that many "experts" are starting to say that the market is going to dump anytime soon. So, with these experts telling from Twitter(X) or from any platform that they're in and as well as with the interview in the likes of Bloomberg and other financial segments in the global televised programs, you'd see them telling the opposite of what should happen. I'm not sure if many still remember that when these "experts" tell that we're going to crash, the market is going oppositely and it goes up and the same if they tell their sentiment about dump, the opposite reaction of the market comes.