With regard to the concept of 'correctness' of individual language elements, there are two competing views and these two views will affect how to differentiate varieties.
There is describing scientifically what is out there, what people actually say and how their individual way of speaking can be categorized with others, and then there is systematizing variation, stipulating rules, limiting varieties out of standardizing communication or esthetics.
The first is called descriptivism, the second prescriptivism. Prescriptivism is very much centered on recognizing a single standard; phenomenon X is either 'correct' or not with respect to that single standard. Descriptivism is much more lenient with respect to alternatives and varieties; many varieties can be recognized. Different social registers, ethnicities, localities tend to have within their group consistency such that phenomenon Z is indicative of someone being in a particular group (descriptivists could conceivably use the term 'correct' just allowing many more variations or alterations within a variety). For either -ism, there is a concept of 'this is a phenomenon, but that is not'. However the term 'correct' is socially laden with judgement of other varieties.
'Correctness' is the term used prescriptively for a given standard. Because scientifically there are general groups of people, there are many possible standards. There is a explicit social attempt at imposing standards (dictionaries, schools, media editors, government sanctioned academies).
Many people will say that an Appalachian dialect is not 'correct' when writing newspaper articles, but using "ain't" is 'correct' or 'preferred' when speaking in that variety.
It seems strange to use the word 'correct' for an entire dialect. Is Australian English 'incorrect'? Of course not. Is Appalachian? Well, if you're writing a paper for a conference, it's probably a poor choice for understandability by the conference population.
A dictionary entry displays what should be said and is very likely the most common method for a particular variety (for most English dictionaries, it will be either American or British English).
To answer your question more directly, it feels inappropriate to use the term 'correct' for a given dialect out of many. The preferred way of saying it is 'This variety is preferred' and 'Phenomenon X is by far the most common and is accepted as standard (for this particular variety)'.
If you are a school teacher or newspaper editor (who normally only deal with one variety), then you can say 'Phenomenon X is correct'.
