There is no easy way to tell unless you can get a spectrum analyzer. I used to use a channel/frequency map on my DTA, plus a frequency shown on my regular cable box in a diagnostics screen, plus frequencies shown on my modem to build up a map. Even then, it's not easy to tell. You may want to use a low-pass filter and put your analogs up high, as long as you put it on a branch separate from your modem. Most Comcast areas tend to use 700MHz+ for OFDM, which is used by DOCSIS 3.1.
And even after that, it can change. Many years ago, before they dropped analog, I used to use some open frequencies for a feed. But, they would move things around and I would get intereference. I eventually used a notch filter. I no longer do any channel injection.