They’ve learned a thing or two about relationships from spending their nights and days with couples — long-term couples sharing drinks, first-time dates sharing meals, madly in love couples sharing the same side of a very small booth.

“We’re like front-row seats on a relationship,” said Christine Cikowski, co-owner of Honey Butter Fried Chicken, in Chicago.

Without really trying, servers notice when you size up a date you’re meeting for the first time. They glimpse you holding hands across the table. They catch you checking your phone a little too often.

That’s why we turned to servers — from those who work at the hot spots to those behind the tables at eateries — for their advice about the ingredients of a healthy relationship.

It turns out that the qualities that make for a pleasant customer — patience, listening skills, good manners — make for a pleasant partnership as well.

“When you have a guy who sits down and automatically asks you for a Bud Light, which is not even on the menu, you know you’ve got someone who just likes what they like,” said Jillian Jackson, a server at the Promontory. “That’s a different experience than having someone sit down and listen to what you have to say and ask a bunch of questions.”

Paying attention to your partner is key. If Jackson sees one of the pair checking her phone constantly, she worries about the relationship.

“A couple should be talking,” said Jeff Lawler, owner of Geja’s Cafe. “They shouldn’t be on their phones. Even if it’s in a McDonald’s, they have to be present for each other.”

Other cautions: “one drinking way more than the other,” said Jackson. “If he’s on his second Manhattan to her first glass of cabernet, you sort of wonder.”

Laughter’s an obvious good sign. As is talking, unless one person is doing all of it.

Other good signs, the servers said, include hand-holding, eye contact, no phones on the table and a drink (alcohol or coffee) after the meal.

The servers agree that first dates are always easy to spot.

“You know it’s an internet app date right away,” said Jenny Lovin, a server at Masa Azul. “One person shows up before the other and there’s a standup and awkward hug when the other person comes in.”

Then you have the couples who’ve been together for ages.

Wait staffs see — and learn from — them, too.

“Nothing makes me happier than the sweet, in-love older couple, holding hands,” said Jason Lerner, Masa Azul’s owner. “For me that’s the dream, right? That’s where we all want to end up. At that point, for me, they can do whatever the heck they want. They can sit on the same side of the booth. They can make out. They’ve earned it.”