Listen to highest frequencies, for example cymbals and other instruments that have frequency content spreading beyond 22.05 kHz. High frequency transients are the best. Listen for accentuated glare and unnatural “rough, digital brightness” or extra sheen or shimmer at the top end. Instead of precise, fast, clean and smooth sound you get from apodizing filters.
Sometimes one could initially mistake such extra brightness as extra detail, but it is actually the contrary, it hides the actual details in overwhelming sheen and shimmer that “blurs” the top-end. A bit like scratchy car windscreen in a direct front sunlight.
When Apod counter increments, there are certainly error signals in the source data that don’t belong to the original analog signal. These are digital artifacts that are results of the DSP algorithms in the production chain, either (typically) from the ADC itself, and/or from the production tools, such as ProTools.
Non-apodizing filter will reproduce these artifacts, so you are listening to the original signal plus digital errors etched on top.