I have never been able to swallow the explanation that this means οίσθ' ὃ τɩ ἒστɩ τ⋯ν κακ⋯ν ⋯ποῖον κ.τ.λ. The order of words is dead against it, since Zεύς ought to be in the ⋯ποῖον clause; it may be safely said that there is no shadow of a parallel to such an order in Sophocles, and probably not in anyone else either. Look at line 2 by itself and consider whether any hearer could possibly suppose that δτɩ Ζεὺς could mean anything but ‘that Zeus’ is doing or will do something or other. So Aristophanes seems to have thought; in a passage crammed with reminiscences of tragedy he declaims (Birds 1246):

ἆρ' οἶσθ' ὃτɩ Ζεύς εἳ με λυπήσεɩ πέρα μέλαθρα μ⋯ν αύτο⋯ κα⋯ δόμοѵς' AμΦ⋯ονος καταɩθαλώσω πυρΦόροɩσѵν ⋯ετοῖς;

Moreover, I flatly deny that οἶσθ' ⋯ τɩ κακ⋯ν όποῖον ού τελεῖ is possible Greek even when the order has been corrected. οὐĸ οἶδα ὃστɩς τ⋯ν ‘Eλλήνων όποῖος ούκ ᾀπαρνοῖτ’ ἂѵ τοȖτο μ⋯ οὺκ εἶναɩ ⋯ληθ⋯ς. Just look at it!