The DM style matters a lot here, and more proactively descriptive DM's are easier to work with when it comes to 'calling' your portent
I have played with DM's who will roll, decide the outcome and then narrate, for example:
DM: "Does a 15 hit?"
Player: "Yes"
DM: "The enemy hits you and it hurts, take x points of damage"
If you have a DM who does this it is very difficult to know what is going on in enough time to call your roll. I have played with such a DM and had to have a conversation with him because this style is either a huge nerf (you never get to use portent) or a huge buff (you can use portent after knowing the roll).
In the end the DM could not adapt their style so I ended up just calling portent even after the roll, and asked for the DM to trust that I was playing fairly.
Other DM's try and give descriptions of what actions are being taken, and then roll to determine the result, for example:
DM: "The enemy is going to try and hurt you"
Player: "Aww why me DM, you are not fair! You are the worst DM ever!"
DM: "Does a 15 hit?"
Player: "Ha no it doesn't, in your face DM!"
This style is much easier to work with because you know what is being rolled, and why, before the dice clatter. This gives you the chance to think and call portent early, and allows the power to work as intended.
There are way more DM styles than I can call here, but these are 2 of the main types and show the issues or benefits with how styles can interact with portent. There is no "how DM's normally handle it" because each DM is different, but I find usually once a DM settles into a style it is very difficult for them to accommodate things like portent if their style doesn't already fit. If you have one of those DM's you need to have a conversation with them and come to a solution, which could range from 'suck it up', 'DM will adapt', 'you can call even after knowing the roll', or 'well I am going to be an illusionist instead', and various options in between.