Definitions will vary from person to person, and plenty of games in each camp will represent some but not all of their defining characteristics, but you'll see some common themes. Historically, I've also preferred western RPGs by a wide margin, so that might color some of my definitions below. Also, both of these branches in RPGs had the same starting reference of D&D, and then a multi-decade game of whisper down the lane led to them diverging more and more.

Western RPGs:

• character creation, choosing from classes that you'll often see represented by other NPCs
• allocating attribute points, both at character creation and as you level up, that govern other things about your character
• generally flatter power progression (you might do hundreds more damage at the end of the game than you do at the beginning, but not hundreds of thousands more damage)
• in attempts to recreate the tabletop experience, will often times allow for outside-the-box solutions to problems besides combat as well as choices that affect the world state

JRPGs:

• usually a finite cast of characters that level up more or less only in one way, but you might have a secondary system for them to customize with equipment beyond weapons and armor
• combat usually doesn't involve positioning on something like a tactical map but rather a line of combatants on each side of the screen
• magic and abilities are more often limited by a magic points resource instead of a rest system
• dialogue with NPCs tends to be more limited in choices, telling a more linear narrative

I'll be honest, trying to differentiate these two with a list of bullet points was harder than I thought it would be to articulate. I'm almost more inclined to just say "I know it when I see it", haha. But for some points of reference, I'd say Baldur's Gate 3, Pillars of Eternity, and The Witcher 3 are western RPGs; Final Fantasy VII, Persona 5, and Pokemon are JRPGs; Sea of Stars is a JRPG that isn't made by a Japanese developer; and while also an action game, Dark Souls is closer to being a western RPG than a JRPG.