So many slang words that assume nobody trusts you
Honestly, if one more person says that something is "actually" amazing, I'm literally going to die. Seriously. It's like they think that I'd think they were lying out of kindness, or for whatever other reason that I'd think their endorsement were untrustworthy. But be assured, that new reality show about hairstylists who compete for job slots at a doggie day spa -- is "actually" pretty amazing.
"For real" started spreading sometime in the mid-1990s. I don't remember hearing that in elementary school, and I have a pretty good memory of '80s and early '90s slang, so I'd guess it was around '93 or later. Same with "the real deal". In 1991, MC Hammer put out a song called "Too Legit to Quit", although I've only heard "legit" (i.e. legitimate, authentic) used among young people in the past 5 years. I guess it was a little ahead of its time.
"Seriously" I can't date from memory, although it's not '80s. I'd say it was there a little later, maybe late '90s or after. "Literally" also sounds like the late '90s or later. "Actually" is from the 2000s, which also saw the rise of "honestly," "to be honest," and "to be quite honest." The genre must have reached its peak with "I'm not gonna lie", another one from the 2000s.
All of these qualifiers reveal a deep anxiety that your conversation partner would normally think you're a liar, not to be trusted, etc. So you have to assure them at the outset that you're not telling a lie. There are too many different examples, and even multiple variations on a single phrase (the "honest" ones), for them all to be random flukes that just happen to mean the same thing. The General Social Survey shows that trust levels peaked in 1989 and have been falling since, so it's no stretch to see the slang words reflecting broader social trends.
The only '70s and '80s slang on the theme of trust and belief was the family of incredulous expressions -- "No way," "Not even," "Get serious," and "Get real". ("Inconceivable!") You didn't say these things because you thought the other person was lying and dishonest, but that their claim was unbelievable for some other reason. Maybe they were unbelievably naive to make such a statement, or maybe the event seemed so improbable that you couldn't believe it truly happened.
If the expression of disbelief (but not of distrust) touched a nerve, then the other person might water down their claim. But if the claim really was to be believed, they assured you. "No way" -- "Way!" "Not even" -- "Even!" "Get serious" -- "Seriously!" "I don't believe it" -- "You better believe it!" In these cases, you made an overt assurance of truthfulness only after being explicitly told that your claim sounded unbelievable. It wasn't the default mode, before any expression of disbelief from your conversation partner. It was not pre-emptive, as though everybody would distrust you.