Dialectical materialism is a model for a particular relationship that exists between certain subjects of the natural and social sciences. It is not falsifiable in the sense that mathematical definitions are not falsifiable - you just cannot encounter a circle containing two points that are not equidistant from its centre. But though cannot falsify dialectical materialism itself, you can enquire whether different entities have a dialectical relationship among each other. If that is the case, you can predict, with some degree of certainty, their future co-evolution. If the prediction turns out to be false, well, then that relationship likely wasn’t subject to dialectical materialism.

If you want an analogy, think of the circle again. Despite no scientific inquiry being possible over whether perfect circles actually exist anywhere in nature, they have proven useful for modelling physical objects. By assuming them as circles, we can come to conclusions that are very useful and workable, almost bafflingly so. If an object that we have previously modelled as a circle later turns out to exhibit properties we did not expect, like planetary orbits, then it is likely not a circle, but rather from a broader, related class of shapes - ellipses. However, the geometric theory we derived about circles still holds, it is just this particular object that has an issue being modelled as one.