Cas gauges danger by how much Dean flirts with his sister.

Low priority: Dean shows up, flirts with Anna, is eventually cajoled into telling them why he showed up ("What, I can't just say hi to my favorite hunters?"), then continues flirting with Anna afterward.
Medium priority: Dean shows up, flirts with Anna a bit, gets to the point. If he doesn't continue to flirt with her afterward, bring extra ammo.
High priority: Dean shows up, tells Anna she's looking good, and gets to the point.

So, one time, when Anna goes off to get more salt and Cas stays in the motel room doing last-minute lore research, he asks Dean, "Have you ever had sex before?" because he is curious. "Do angels have sex?"

Dean says, "Woah, dude."

"It's just that it seems you wanna have sex with my sister."

"I've had sex before," Dean replies.

"With my sister?"

"Uh," Dean says carefully, because angels can't lie.

Cas raises one eyebrow. "What the hell kind of angel are you?"

"A damn good one," Dean retorts.

"Uh-huh..."

"Hey, I don't have to explain myself to you, meatbag."

Cas shrugs. "I didn't say you should."