>>96766499
So, within the universe, people rarely know what is being cast. To truly figure out what's being cast, you need to recall knowledge as the spell is being cast in the moment. When someone casts a Spell, they don't say
>Warlock: "ELDRTICH BLAST!!"
They most likely say
>Warlock: "Tdll'hc Ebi'ctab-rs!"
Which means nothing to 99% of people, then the spell itself happens. And the spell itself can then look completely different for each spell caster, one could be a magical blast of flame like force another could look like ribbons, while another could like swords and axes.
This gets worse when you factor in that not everyone casts 'spells'. Some creatures can make attacks that could look similar to certain spells.

All's to say, magic is so personalized that it borders on impossible to know exactly where the origin point of the magic comes from without explicitly knowing the vast types of spellcasters in the world. And even then, you could still make a mistake just because the effect you're examining looks similar to something else.

>wouldn't any knowledgeable mage be able to see an Eldritch Blast being cast and instantly be able to tell that "hey, that person must've made a pact with some kind of being" or something like that?
In 5e D&D the best case you could get for this- would be having a really high Arcana skill stat, taking a Reaction to identify the spell being cast in the moment (Note it must have components that you can see), then you can determine what it is on a success, from there if your character is aware that the Eldrtich Blast spell could only be obtained from a Pact. Then yes, but if they don't know that then they won't!