Check if a large matrix is diagonal matrix in python

I have compute a very large matrix M with lots of degenerate eigenvectors(different eigenvectors with same eigenvalues). I use QR decomposition to make sure these eigenvectors are orthonormal, so the Q is the orthonormal eigenvectors of M, and Q^{-1}MQ = D, where D is diagonal matrix. Now I want to check if D is truly diagonal matrix, but when I print D, the matrix is too large to show all of them, so how can I know if it is truly diagonal matrix?

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