How do you know you love someone and not the idea of who they are?

When you see their flaws and you still feel the same way. When you see them at their worst and you don't shy away. When you see their baggage and you don't run away...

Amongst other things 😅

spend time with them

You don’t. That’s why it’s important to continue to get to know someone even after you know them

Love yourself then you wont need an "idea" of who others are because you will be enough.

What JupiterDreams said.
It's seeing them in their true humanness, during conflict or more negative human emotions, beyond the sweet romantic accolades.

Be with them in bad situations, see them angry, smell their farts lol.