In Breaking Bad season 2 we have seen that Walter goes to meet Gus Fring in Los Pollos Hermanos, but exactly how did Walter White identify that Gus Fring who is the Restaurant manager is the guy whom he is supposed to meet for meth business?
Here's some relevant dialog from S02E11:
Walt: “We’re not going to deal with another high-level distributor. We’ve been down that road.”
“What? Some tattooed speed freak? No. What you two need is an honest-to-God businessman – somebody who treats your product like the high-margin commodity it is. Somebody who ships out of town, deals only in bulk – someone who’s been doin’ this for 20 years and never been caught.”
Walt leans forward. “You know someone like that?”
“Let’s just say I know a guy who knows a guy, who knows another guy. Let me make some calls – see if I can get a meeting.”
“What’s his name?”
Saul, back at his desk: “I have no idea. He’s very low profile – he’s careful like that. From what I do hear about him, he sounds a little like you.”
When Walt and Jesse are in the restaurant, Walt observes everyone.
We get our first sight of drug kingpin Gus Fring, in a yellow shirt and glasses – humbly, quietly, and efficiently working. Walt’s in the restaurant, too, looking around with narrowed eyes and grim, thin lips, scrutinizing every man in the place.
While they're talking, Gus comes over:
“Gentlemen…Is everything to your satisfaction?”
“Fine. Thanks,” Walt says. Gus leaves.
That's the only contact Gus makes.
Walt calls Saul afterwards:
Walt uses his second cell phone to call Saul from his classroom. “He wasn’t there,” he says.
“Oh, he was there all right.”
“What do you mean? I sat there for two hours – nobody came.”
“All I know is, my contact said he was there. Are you sure you were at the right restaurant?”
“Yes. I’m positive. Where was he?”
“I don’t know. Maybe he sat out in the parking lot, took one look at you, and decided he didn’t like the cut of your jib. Anyway, it doesn’t matter now, because my contact says it’s a no go.”
“What? How can he make that decision without ever meeting me?”
“He’s very cautious who he does business with.”
“Call him back.”
“Hey – first of all, I never called him. I called a guy who called a guy who called a guy. Second of all, it’s over. Understand? With this particular individual all you get is one shot… Look, I’ll see if I can scrape somethin’ else up. But don’t get your hopes up. No one else handles that kind of bulk.”
The next day, Walt returns to Los Pollos Hermanos.
Walt eats, and time passes. Finally, Walt gets up and goes to the counter. “Can I help you, sir?” Gus asks.
“A Diet Coke, please, and five minutes of your time.”
Gus brings the soda to Walt’s table. “What can I do for you?”
“Have a seat. Please…I would like to know why you wouldn’t meet with me yesterday.”
“I’m sorry – I’m not following.”
“I sat here yesterday waiting to meet somebody. I believe that person was you.”
IMHO Walt is pretty much sure that Gus is the person he was supposed to meet, but he cannot reveal too much.
“I think you’re confusing me with someone else.”
“I don’t think so.”
Gus turns and looks at the counter. “Sir, if you have a complaint, I suggest you submit it to our e-mail system. I’ll be happy to refer you to our website.”
“I was told that the man I would be meeting with is very careful – a cautious man. I believe we’re alike in that way. If you are who I think you are, you should give me another chance.”
Same as before. Then finally Gus admits he is the person who Walt was supposed to meet the day before:
“I don’t think we’re alike at all, Mr. White. You are not a cautious man at all. Your partner was late, and he was high.”
The key scene is when Walter sits down at the restaurant, looks in the window (it's dark outside) and sees the reflection of him, and Gus. Both have short (or no) hair, and are wearing glasses, and Walt realizes that Gus fits the description.
Walt realises the moment when he sees Gus's reflection in the window. He looks at him for a long moment and then it occurs to Walt that this was the only person they met yesterday. Possibly this person could get a good picture of them as opposed to some guy looking at them through the window of a car. Walt saw that whoever this man was, he was really meticulous and had the image of a perfect businessman. He just knew.