The most frequent criticism made of any claims that Freudian analysis is valid as a science is that it's non-falsifiable, which is to say that a Freudian logic of denial, along the lines of: analyst tests hypothesis H, patient denies H, analyst concludes H is true, does not allow the falsification of the hypothesis.

In a science, we expect there to be some way of showing our theories are incorrect - a scientific hypothesis is first of all a falsifiable one, and if it doesn't have this property, it isn't supposed to count as a scientific hypothesis. That's not to say that scientists don't see their experiments as confirming, rather than 'failing to falsify', their theories, or indeed introduce extra theories to explain the falsification, before they give up their theories, but that the logical structure of scientific knowledge is grounded in experiments, predictions and measurements.

Psychologists, in the main, are very conscious of this. At the extremes some (eg Skinner) have sought to entirely eradicate the study of the mental from psychology in order to qualify as a science. Indeed it's true that (assuming no epochal revolution in neuroscience has taken place) the only access to the mental a science can have is by inference from behaviour - though modern attempts like cognitive psychology are willing to infer quite a lot over and above the bare stimulus-response black box implied by Skinner's views - so psychology is in a uniquely difficult position with regard to its subject matter.

This is why psychologists are usually quite careful to frame their hypotheses in ways that can be falsified by the experimental data they collect.

This might not sound like much of a difference, but it leads to an entirely different approach to knowledge in the two fields. An analyst must undergo analysis, and take patients, in order to be thought competent. A psychologist needs to submit papers which may be criticised on the grounds that the falsifiability criterion is not met. You won't find that criticism being made in learned Freudian journals.

This is enough, I think, to demonstrate (without actually establishing the objectivity of psychology) that the two fields really have completely different approaches to the study of the mind. We should at least agree not to blame one for the sins of the other.