Anyone know how to determine bitrate/res on Stereophonic?

Without buying equipment, I mean.
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I know about bitrate but don't understand your question. If its Mp3 it will usually say that on playback. You can look at the file size and the length to also get an idea. Lossless is mostly Flac or M3a  for PC. I believe it is about 768>. Cant remember Apple although M3a was originally Apple. I haven't used Mp3 in many years so I forgot the formula. I know that flac is generally about 10 megs per minute. If I were to guess, 320 is somewhere around 4-5 megs. Hope it helps

OP probably means, a classical only service  They transmit in 44.1/16 or 48/24 but their streams are unmarked.

Your need a program like MusicScope, which is no longer available, to see what they, or any other streamer, is actually sending.  My own inquiry suggests that Primephonic, unlike several others, actually delivers what they promise.

By the way, file size (FLAC or otherwise) is no indicator of SQ.  It only gives you the size of the container, not of the non-zero bits within.