I calculated the correlations between ROT13’s expected ciphertext frequencies for Vigenere and Variant/Beufort and the actual frequencies of the 10,000 Gromark and running key ciphers that I generated. (For each cipher I used the higher of its Vigenere and its Variant correlation).
Here’s a chart of the correlations. In general the correlations are higher for running key ciphers than for Gromark ciphers. (but there were 258 running keys where the correlation was negative!). It looks like a correlation higher than 0.3 is a reasonable indicator for a running key cipher. But between 0.2 and 0.3 the odds are about even between Gromark and running key.