Not if you inform the customers.
My iPhone has WiFi turned off most of the time. The only time it is on is when I turn it on to connect to particular WiFi access points (at home, mostly). So, even if I visited your shop, you would not find out about my phone.
I'm not sure when a WiFi packet contains any useful information. That may be when the phone is trying to access a WiFi access point (all the time it is looking) or it may be only when the access point agrees to let the device connect.
Only one way to find out.