The phrase "able to discern when" is correct and usable in written English.
You can use it when discussing the ability to recognize or understand specific situations or moments.
Example: "She is able to discern when her friends are feeling down and offers support accordingly."
Alternatives: "capable of recognizing when" or "able to identify when".
John Norseen, a scientist with Lockheed Martin, is often able to discern when subjects are thinking of particular numbers.
I have no intent to use it and I am easily able to discern when someone is using that appropriately, and I think the same applies to the "N -word.
She had a compulsion to give, and this meant that she wasn't always able to discern when to step in and when to step back.
The key is being able to discern when a less-bespoke, less-costly solution is good enough for your needs and moving on to tasks that will create more value for the enterprise.
For a police officer, wisdom is being able to discern when gentle compassion is called for and when fierce compassion is called for.
From the pedigree information, we were able to discern when a pedigree must have had a male European founder as well as their estimated date of arrival on the island.
Of course, we're much more able to discern that when we allow ourselves to be in our right mind.
Your earnest inability to discern when someone is teasing you?
It is only when children are educated by materials that are written on good authority to be truthful that they develop the prior knowledge to be able to discern false news when they hear it.
Were an intelligent, determined scholar to set herself the task of sifting through the last thousand years of Western culture's literary output, would she be able to discern a "historical moment when something changed in the way women's love was experienced and interpreted"?
Not being able to discern how fatty a food is when mildly depressed or anxious has obvious health implications in that it could, as it says in the study, "foster unconscious eating of fatty foods".
