1. He's always trying to match them up with his friends to the point where you feel like you're on some weird dating show. This is the highest compliment. If he wants one of his friends to meet them, it's because he likes them.
2. He always gets you the exact Kate Spade bag you wanted, and you find out later it's because he asked your friends. Not only is he voluntarily interacting with them voluntarily, but he holds their opinion in high regard. Or he just has no idea what to get you and needs them to remind him that you really wanted a new purse.
3. He's totally fine with a night in when you want to go out with them. If he didn't like them (or if he's jealous) he'd probably complain that they're "stealing" you from him. If he wants you to go out and have a good time with your friends, he trusts them and he knows its important.
4. And he doesn't care if they ever tag along. More telling is if he's fine on turning "date night" into "let me bring my one friend along" night. If he doesn't think that ruins your time together, he must like your friend.
5. He talks about how well you get along with them with the same reverence he talks about McDonald's breakfast. He's impressed with how long you've known each other and how deep your friendship is. It doesn't matter if he isn't best friend with your best friend; if he sees how great the friendship is, he can still approve.
6. He doesn't pretend to fall asleep when you ask him to hang out with them. If plans suddenly come up when you tell him you're all going out, he's trying to ditch (no one "suddenly remembers" that they've got plans in an hour). But if he's still excited to tag along on girls night, he must get along with your friends. Trust me, no one would ever like girls night otherwise.
7. He has his own inside jokes with them and it's actually getting kind of weird. You realize one day that he's really developed a friendship with them independently of you. You're a little jealous, but you're mostly impressed.
8. Sometimes he knows about things in their lives before you do and your gut instinct is that he's psychic. They actually talk to each other on occasion. Not like, "maybe-they're-having-an-affair" constant talking, but he happened to know about your friend's promotion because your friend group texted both of you.
9. He knows exactly who your bridesmaids would be. Not that you're necessarily talking about marriage, but he doesn't just know your friends, he knows all about how you feel about your friends.
10. He always asks how they're doing. He doesn't wait for you to tell him how your friends are doing, because he actually wants to know. If he didn't like them, he'd never purposefully subject himself to a story about a person he doesn't care about.
11. He posts dumb stuff on their Facebook walls. Is this the greatest gesture of friendship possible? No. But posting Vine Videos on each other's Facebook wall is the millennial equivalent of a cordial but brief letter of correspondence.
12. He's disappointed when they don't show up to a party. Maybe even more than if his own friends don't show up.
13. He doesn't try to be someone else in front of them. This is the biggest tell. If he feels like he can act himself and doesn't have to put on a front, he's legitimately comfortable around him.