"player 1 after that attack your character is aware that that weapon is ineffective as it bounces off the thick hide/the wound heals as fast as it's made/as it's hard to hit more then the air with a point/etc"

"Player 2. Your spell envelopes the target area settings everything on fire, but standing in the center of the blast is the target completely untouched by the magic fire."

It's ok to tell the players something their character would know from their life experience in very clear terms. A trained sword fighter or spell slinger will have used their weapons often enough they and failed enough to know the difference between worked/worked but not as expected/ineffective on a successful hit.

This is helpful because you're more likely to have a player try to identify the creature, and if not you're more likely to avoid TPK and hurt feelings from wasting resources trying the same thing over and over.

Example a demonic iron golem who has damage resistance 30 against non magic and non adamantium weakness 15 to holy.

Barb attacks with an adamantine axe. Hits and deals 40 damage. I would say: "your attack hits, and the blade scrapes the steel of the creature, but doesn't penetrate like usual. A glimmer of the weave tries to resist the attack but raw power allows some damage, but this axe is not very effective."

Rogue strikes with his 1 dagger. Hits deals 22 damage. "the weave flickers as you dagger strikes, but the metal of its hide is stronger than your blade. It can't cut as deeply as it should. More force could help, but his attack didn't do anything. This dagger is not very effective.

Cleric rolls for more information, and casts precious metal on the rogue's dagger and rune weapon on the barbs axe.

Wizard casts fireball.