Depending on the game, it can be a straight-up guess. That’s just the bottom line. Above a certain level of gameplay, there’s no real need to put any more thought into it (you end up with the "I know that he knows that I know etc “mindgame”). That’s why you can see Daigo getting thrown to death in 3S, and him throwing other people to death in SFV.
Games that have strong throw/attack mixups are usually centered around the neutral game strategies that you should be using to avoid getting put into that position. So if you are playing a game like this, the question you should be asking is how to not get caught in that situation where you are forced to guess.
Here’s a well-known set between two good ST players:
It’s kinda pointless for the Guile player to ask how to guess between low and SPD. Instead, he has to change his neutral game strategy so that Gief can’t put him in that situation in the first place.
Having said that, there are games where neutral strategy isn’t that important and being able to guess better than the opponent up close is an essential part of gameplay.