I only have experience with 3|0 blitz so I don't know anything about that.
You also get a sense whether you should adjust up or down from the norm based on their average opponent, highest rating, and their most frequent opponent... stats like that.
For example one way to boost your highest rating is to play a long series against an opponent you're doing well against. Usually a player near your rating who is having a bad day, and you're beating the crap out of them.
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But if a person's highest rating is 200 points higher than their current rating, and they haven't played a long series of games against anyone (and that peak wasn't made during their provisional period) then you might assume they're currently underrated.
How can you compare OTB ratings at much slower time controls with blitz chess?
Because a lot of people play both, and they have similar ratings.
For example I know 3 or 4 players rated 1900-2100 who play blitz here, and their chess.com blitz ratings are... 1900-2100.
It's also easy to check people's profiles and over time you get a good sense for these things. Also a few times in the past people have compiled data for a few 100 users and put it into a table and graph.
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It's too bad there are so many garbage answers (not you, now I'm just talking in general about this topic). The OP just wanted a general idea. I did my best to answer in #20.