Notice if she asks the same personal question each time. Many times Customer Service is instructed:
Greet the customer by name
ask pertinent questions to the transaction
ask one personal question such as "have any plans for the weekend?", "are you having a good day?" "How has your day been?"
Now my cousin asked out a bank teller and now he's married to her so it can work...
You need to assess if her attention is genuine or because she has to to meet her job expectations. You can do that by asking her about what she is doing this weekend. If she is married or has a boyfriend, she will probably mention doing something with him or her family. This is a clear STOP sign.
If she is open, she will volunteer personal details. She will have a nickname for you. She will make herself more available to you than other customers, or make them wait while interacting with you beyond the transaction.
Steph was my bank teller at Globobank. She had personal photos on her computer screen and turned it to show me while I was at the counter.. She called me "that Wrestler looking dude" to her colleagues and still greets me with "hey good lookin'" and a hug when she comes to my place of work. If I wasn't married at the time , she would have gone out with me..
Obviously don't try to get her number when it is busy and there is a line of people out the door. Best time is when you are the only customer and her supervisor and colleagues are away at the desks behind the counter, trying to look busy. You want to not put her on the spot in front of her workmates. If you take the convo in the direction of cool bars, restaurants or coffee places nearby you can seed the concept of you both meeting there. Either during her lunch or after work.
"where is a good place for a coffee nearby?
"The CUPPA JOE on the corner has great cappuccinos."
"Oh yeah? You go there much?
"When I get a 15 minute break at 10:30"
"Super, If I order one for you today it will be ready when you walk in the door. We can talk more about German Shephards. "
Then get her number at the coffee place and seed the second date.
Alternately
You can either slip her your card with your cell number or take one of the counter cards and ask if she has a direct line . If she is just a teller, she probably doesn't and if she is interested she will write down her cell. if she says "sorry I don't but the main line for the branch is there" then you have been politely told "NO". If the Personal banker for the branch calls you to ask if there are some other services you would like to ask about, you have been told NO.