Properly encrypted data is indistinguishable from random noise - aka ‘hiss’. If really good encrytion, it will be white noise (generally). Albeit will have more power.
If there is a clear pattern to it, then that’s either unencrypted framing, or bad encryption. (Think 90’s cable TV ‘scrambling’).
Not really true on modern digital radio systems. They are AES-256, but the voice frames are encrypted right after the vocoder does its thing, then the voice data is dropped into the stream just as if it were clear voice. It's all wrapped in the same same digital protocol (like P25 or numerous others), so the signal is very distinct in that encrypted and clear communications both sound the same to someone listening to the raw audio.
If there is a clear pattern to it, then that’s either unencrypted framing, or bad encryption. (Think 90’s cable TV ‘scrambling’).