You can look at the symbol of the ship to see if it is a battleship or frigate or cruiser.
You can also look at the symbol of any effects those ships apply to your ship like neutralizers or warp disruption. You can even ctrl-click those effects to lock the threat that applies that particular effect to your ship.
If you get some experience and pay attention you can determine whether a frigate is a bigger threat to you or if a battleship is what you should kill first. This depends on many factors: your ship, your weapons, your strategy, the site, etc.
Symbol on hostile NPC that deals a lot of damage to you is the wrong approach.
Do you see that webifier icon on your ship? It deals zero damage, but massively increases the damage you take.
Do you see that target painter icon? Same thing.
Neutralizer? You might want to kill that because if it shuts down your shield repairer this too ‘deals more damage’ than any other ship on the grid.
In other words, even if the game was changed to add a brainless-mode icon to ‘the ship that deals most damage to you’, this ironically often doesn’t point to the ship that you should kill first to reduce damage you take.
And what about new arrivals? What about the battleships that dealt a lot of damage to you while a Titan just spawned? The icon of ‘biggest threat’ would be on one of the battleships as they have been shooting you, suggesting you to brainlessly shoot one of those. That is, until the Titan locks and one-shots your ship.
The main problem described above would be the same though.
the hardest hitting ship is often not hitting that hard because of its base damage and other numbers like accuracy alone on its own but due to effects applied to it or you by other ships.
if you fly a speed-tanking ship and are webbed to hell by frigates and suddenly battleships start to apply heavy damage to you, you can start killing the battle ships, sure.
But taking out 5 Battleships would take waaaaay longer than killing the 1 or 2 webbing frigates which are slowing you down (after which those battleship would miss you 99% again)
I have a tendency to overtank, so I didn’t learned to spot which ones do more damage, but which ones eat my drones. That would be the “(centsomething) Loyal” for Sanchas, “Arch” for Angels, “Dire” for Guristas, “Strain” for Drones NPCs, etc.
I suggest you look at the combat anomalies report on the Eve University website, and spot the “Elite (ship type)” for each wave, to learn their name patterns.
I don’t know which level combat anomalies you do, in what region, but in highsec the “ Forsaken Hideaway” have those drone eaters, if you want some examples.