That's generally shown by the enemy kind of... imploding a little? Like, you can see their body crumpling a bit, there's some explosions effects, some Haki lightning. Blood spitting out (Luffy only made Kaido bleed from his punches once he gained Ryou, but didn't start doing real damage since he added in CoC).
That kind of white blast effect you see coming out of Luffy's fist is supposed to be signaling it, too.
Look at his original attacks on Kaido without Ryou:
Kaido isn't having his body crumple, just move around, he isn't spitting out any blood, the only similarity is the explosion effects.
Now, here's Red Roc, his first Ryou attack on Kaido: you have the same explosions we see with Sanji vs Queen, but, this time, Kaido is crumpling, and bleeding.
It's the combination that makes of those elements that makes it Ryou, but, honestly, I think Oda did a pretty bad job in the Manga of showcasing when characters were using Ryou. I wish Oda was fucking clearer about when characters were using Haki, and what Haki they're using.
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Body crumpling + huge explosions effects + spitting out blood + explosive force = Ryou from Sanji. That's not just fire: that's a detonation. He combined his flames with Ryou Internal Destruction to make Queen a rocket man and launch him all the way to the mainland.