In Avengers: Endgame, after
Loki makes off with the Tesseract in 2014,
Tony Stark says to Steve Rogers that he knows
when they were both there, and you know why I know
Referring to Hank Pym and Howard Stark.
Then the two of them
travel back to 1970 to obtain the Tesseract and more Pym particles from Howard Stark and Hank Pym, respectively.
What was Tony referring to when he said this to Steve?
The problem they have is that they have only one more Pym Capsule each, so if they went back in time to get the Tesseract again, they wouldn't have the Pym to get back to 2023.
Tony says that a stroll down the memory lane brings up "Military installation, Garden State.". It's a time and place that his father, Howard Stark, was working in the same facility as Hank Pym. It's at a time well before Hank quit S.H.I.E.L.D.
Tony says he has a vague idea. It's a bit of a guess which is why he asks Cap if he trusts him. So Tony's guess of the year 1970 is based on the place where his dad worked when he was born and a time when Hank may have fully figured the Pym Particle.
In the first Avengers movie, Tony hacks into SHIELD's computers while onboard the helicarrier by placing a device of some sort on the command terminals used by Nick Fury on the bridge.
Later when Tony, Bruce, and Steve are in the lab working on a way to locate the Tesseract, Tony tells Steve that Jarvis is currently hacking SHIELD's computers and, "In a few hours, I will know every dirty secret SHIELD has tried to hide."
It is safe to assume that this information would have included data on both the history of the Tesseract and Dr. Pym.
It is reasonable to assume that before starting the mission to travel back in time that Tony, Steve, and the rest of the team would have reviewed all of this information looking for places in time where they could locate the infinity stones.
This piece of information, in particular, might have piqued their interest and drawn their attention as it involved; one of the items they were looking for, a place Steve was very familiar with, a place Tony's dad worked, and the Pym particles they were using to execute the time travel plan.
It might even have been on the list of possible times and places to get the space stone but was ruled out as there was a better opportunity to get three stones at one time during the battle of New York.
The fact that Scott Lang doesn't seem to know about this alternative time and location while Tony and Steve do, is a brilliant use of the character driving plot. Tony and Steve are both detailed oriented and would have read the briefings in-depth and committed them to memory. Scott on the other hand, well he's been established as a bit of a slacker and probably would not have read all of the briefing data.
I'm currently watching Endgame on Disney Plus with the subtitles on. Tony and the script both say: "and I know why I know." Not "you know why I know."
I think @MovieMe is right though in his second post. Tony knows to go somewhere around 1970 due to the information about his parents and Bucky revealed to him in Civil War. And he is definitely conveying that through the tone and inflection in his voice. That's why Captain America takes him seriously and knows he'll get it right.
The answer to how Tony Knew how he Knew to go to Lehigh on 4/7/70 is a simple two word answer....STARK EXPO..which Tony viewed video of Expo buildings and Camp Lehigh and read notes and diagrams on during the events of his "house arrest" by Nick Fury during the events of Iron Man 2 this is why Tony says "and i know how i know".Recall Tony listened to his Father Howard say in the video " unlimited energy "and " no more Tedious work (Refereing to the Tesseract( Unlimited Energy) and Pym Particle tedious work( large objects made small to move with ease).both Hank and Howard were at the camp at the same time working on presentations to present at the Stark Expo of 1970 under the protection of the military in plain sight to not arouse suspicion and work in secrecy .
