Is bacterial resistance really due to mutation?

Okay as far as I understand an evolution scientist takes a sample of bacteria in a dish and let it grow. Then they apply some sort of anti-biotics.
When some bacteria survive they conclude that some bacteria must have developed resistance due to mutation as they grew.But surely they haven't ruled out the possibility of resistant bacteria being part of the original sample. If that was the case then there was no mutation in this experiment that caused the resistance - resistance existed all along.How do evolutionists actually know that bacterial resistance is due to mutation and that they aren't simply selecting the few resistant bacteria that already existed in the *original* sample?I've looked for an experiment on this on google and found none.

Good answers.I guess the problem I am having is how do they make these monocultures, or more specifically how do they guarantee that the founding bacterium used is non-resistant. How can they find that out without killing it?I have been challenged on this and while I can find methods online which show mutation is random, or methods that show bacterial resistance is not caused by the poison itself, but I cannot find any method that convincingly rules out the resistant variant already being present at the beginning of the experiment. I have just taken it as a given. You are right it must be true, but how do I convince someone of this?The only method I can think of that would seal it would be to take a large number of bacteria and allow them to found seperate large colonies and then poison them. If resistance is caused by mutation you would expect quite a lot of the colonies to have mutated resistant survivors. If instead resistance cannot be aquired by mutation you would expect only a few of the colonies to have resistant survivors. But this is hardly a nice method of determining it. Surely there must be a better way. Please tell me if you know a way.