"...how did you discover your grandfather wasn’t your bio grandfather? That’s interesting as I would have thought it would be harder to tell which grandfather etc from DNA results."
I'm not the person you asked the question of, but the simplest way would be if there turned out to be unexpected aunts/uncles or cousins that are unrelated to any of your other DNA matches that you can place on your family tree.
When I say aunts/uncles and cousins, I'm talking about people with the typical amount of shared DNA that you and an aunt/uncle might share.
You will typically share around 1,700 cM of DNA with an aunt/uncle but this can range from 1,200 to 2,280. For example, I share 1,526 cM with my aunt who is also on Ancestry.
If a person is your half-aunt/uncle (that is half-siblings of your dad) then you will share less DNA with them, typically around 870 cM with a range of 490 to 1,310.
If you have somebody like this as a DNA match that you do not recognise then that is a good place to start looking.
However, for many people, they only have a few close matches like this. So it is often necessary to look at more distant relationships. Perhaps go down to 100 cM of DNA (this would be something like a second or third cousin, so related to you through your great grandparents or great great grandparents).
You will then need to build your family tree and find out how these people are related to you.
If all of your grandparents are your actual grandparents then you would expect to have DNA matches through each of your grandparents. However, if you cannot find DNA matches through one or more of your grandparents then that may be evidence that that was not the biological grandparent.
Or, it may be that nobody related to that grandparent has done a DNA test.
You would then need to look at any DNA matches that aren't related in any way to the ones that you have already got on your family tree (you can see 'shared matches' that you and another DNA match have in common).
If there are no DNA matches at all through one grandfather and a number of DNA matches that are unrelated to the other ones you have on your tree then it is quite possible that the grandfather is not the actual grandfather and that these other DNA matches are related to the real grandfather.
Just to give an example, I have 20 DNA matches with 100 cM or more.
Of these, one is my aunt and of the other 19, 4 are related through my paternal grandfather, 5 are related through my paternal grandmother, one is related through my maternal grandmother and 9 are related through my maternal grandmother.
You will notice that I only have one DNA match through my maternal grandfather at this level, so, if you don't find any then that doesn't necessarily mean that there is no relation. But if you do find a DNA match then you are definitely related to that grandparent.
[EDIT]
An excellent resource for the expected amount of shared DNA is here:
https://dnapainter.com/tools/sharedcmv4