Back in the day, the plastic that wrapped blank tapes was printed with details about the tape inside, including a frequency response chart.

Of course it differed by brand.

It's also worth considering that a lot of tapes were recorded with Dolby B noise reduction, which boosted high frequencies on the recorded tape, to increase the signal to noise ratio. (You had to use Dolby B at the playback end, too, or it would sound too bright.)