Perhaps your company is better off not hiring sales staff that cannot decide between two big buttons, one labelled

PRESS THIS IF YOUR INTERNET CONNECTION IS GOOD

and the other

PRESS THIS IF YOUR INTERNET CONNECTION IS BAD

Or write a small application that can be run from the sales persons' laptop that determines the connection. But anything that bases a decision on a single probe will have to live with the risk of making the wrong decision. Your current connection speed isn't the only you will have 10 seconds from now.