It is *supposed* to be an ability check using your spellcasting ability modifier against DC 10 + spell level. Thus, it should roughly be a 50% chance, given that you'll have a modifier of +4 or +5 and be using it against a 4th, 5th, or 6th level spell. https://roll20.net/compendium/dnd5e/Counterspell#content

Idk how (if) it's implemented in BG3. Is there anything in the combat log when you do this?