Oh, OK, I see your problem.
You are right: there is no obvious a priori way of telling whether an adjective in -ύς is declined according to the ancient paradigm (ευθύς-ευθεία-ευθύ, gen. του ευθέος etc.) or according to the modern one (βαρύς-βαριά-βαρύ, του βαριού etc.). Both those paradigms are considered regular, which is why we didn't understand your question right away.
Class 3, declined exactly like class 2 except for the dictionary form being spelled with an η, exclusively consists of adjectives of color. (δεξής is an exception, and the more usual form is δεξιός, although δεξί is probably more frequent than δεξιό as a neuter.)
Class 5 can be distinguished from class 4 in that its adjectives are stressed on the penult (next-to-last syllable), while those of class 4 are stressed on the final. Again, there are a few (αυθάδης, κακοήθης... plus the whole class of derived words in -ώδης) class 4 adjectives stressed on the penult, but even native speakers are confused by those. The neuter forms of αυθάδης and κακοήθης e.g. are officially αύθαδες and κακόηθες, but most people would say αυθάδικο and κακοήθες. I myself would hesitate about the stress of the genitive plural: I think it is των αυθάδων but των βραχωδών, but I won't be too surprised if I turn out to be wrong (and I used to be a professional translator!)