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Yeah, I figured that you were using a single strobe and had to choose between exposure @ near object (face) vs. distant object (bottle). Since exposure is related to distance from source, this is where you'd either need to use two different light sources, make the objects more equitable in distance or partially block/reduce the portion @ nearest object (face), etc.
Average the scene ... Photoshop>Filter>Blur>Average.
Some histogram readings will give it to you in a data/numeric value also.
It's not really critical to be defined by the number, your eye will pretty well let you know if you have used a lot/little amount of key lighting and are low, medium or high key. It's kinda like having a laser guided nuclear bomb ... does it really matter if you miss the target by six inches? (Props to those on watch.) Don't get hung up on the number criteria, I was just using it as illustration of another way of defining what your eye already knows.
As to your question @ low key vs. underexposed, I think your eye probably knew the difference.
Were the highlights ... ummm ... highlights (vs. muddy mid-tones)?
As to your question @ histogram ... you'll still want your highlights (if your scene has them) to be on the right side. You may have a large gap between the mountain on the left and the small crumbs in the middle before you reach your highlights. But, if your scene has bright highlights (rim lighting, etc.) and they aren't where they should be, you're probably underexposed. Conversely, if you are shooting high key and your scene has blacks, but they aren't located on the left, you're probably overexposed.
Mostly, you'll still want full tonal range (assuming you have both blacks & whites/highlights in your scene), just that your mountain (volume) will be in either the left quarter (low key) or the right quarter (high key), instead of the middle two quarters (normal/average/typical scene & lighting). The histo on your selfy gives a pretty good clue @ how the mountain is on the left, with crumbs in the middle/right, yet still extends full range to the right. (Reverse for high key).