I think even if I did know the DC to pass or AC to hit, I still prefer letting the DM describe the results. Sometimes there are hidden factors that may affect the outcome. I don't want to assume that I know what the NPCs will do or how they will react (I also try not to RP any NPCs, unless given express permission to do so).
Instead, I try to write my descriptions of my attempts in such a way as to help the DM process the result.
"Pressing with his legs and pulling his arms forward, the dragonborn strains against the entangling vines," for a STR save. If I write it well enough, it should give the DM an idea of what I think will happen should I succeed, and can add in any external factors in their description of my attempt.
"Summoning all her magical power, the warlock hurls a flurry of eldritch darts towards her target's chest," for an Eldritch Blast attack roll. The DM might pick up on that and not describe a successful attack as hitting the arms and legs, or head.