You are right, it's impossible. What is possible though is to be aware of your biases and to do your best not to let them interfere with your work (therapy in this case), and this is what therapists learn to do in training..of course, with various degrees of success Unfortunately, I've seen way too many therapists who impose their personal biases on people quite forcefully..

But biases aside, there are certain beliefs about how psychotherapy is best done upon which the entire way of practicing is based. There are as many beliefs about that as there are therapists

All in all, yes, therapists are only humans despite of their education and training, and because of that and also because psychotherapy is a largely subjective field where many methods are based on beliefs, not facts, you won't be able to get the same expert advice as you can get say from a plumber about your leaking toilet or from a mechanic about your car. You won't even be able to get the same accuracy as you'd get from a medical doctor who can base their opinion on the objective tests' results. And, yet, as imprecise as psychotherapy is, you can still get a lot of useful information that you may be able to apply in your daily life. In any case, there is no risk in going to this first consultation and seeing what this is about