I use reversals… Even in decks that aren't designed to be used reversed.
That's perfectly fine. I think all decks can be read upright-only, and all decks can be read with reversals.
My question is this: if you read the cards without reversals and right-side up only,
<cheerfully> Yep, that's me!
how do you determine whether the shadow side of the card applies to its interpretation as opposed to the standard meaning?
Well, they don't have a "standard meaning" and a "reversed meaning". The longer I use Tarot the more aware I am that each card is a continuum of blended and related meanings.
How do I choose? I don't. The right one is the one that pops into my mind first, and allows itself to be elaborated into a story that makes sense to the client (if not to me).
Just very occasionally, probably a couple of times a year, I might go badly, badly wrong, and I'dd know this because I'll start coughing. And develop a fit of coughing that prevents me from talking. This gives me a moment to re-think and change direction slightly. If I do that right, I talk without coughing. But that happens vanishingly rarely - usually the first thing that comes out of your mouth without too much conscious thought is right.
If you can't see the card upside down, how do you know which side of itself it's indicating?
It doesn't have two sides - it has more a blur of meanings. What meaning comes out first? What meaning makes sense in terms of what you've already seen in the spread? Go with that.
Also, with regard to reading reversals in general, I've found that they can be interpreted in five ways - this if from Caitlin Matthews' blog, co-author of The Wildwood Tarot,
Caitlin is good value - I've been reading her books since the middle 1980s.
She didn't mean that there were only five ways to read them. She was giving you an example of how there are more than one, or more than two, ways of reading them. I'm sure she'd agree in an instant that each and every card (including the Sun) can be read in many, many dozens of ways, if not hundreds. I've been reading Tarot for decades, and I don't believe I've said exactly the same things about even one card more than once.
So now I'm wondering, even if you do use reversed cards, how are you supposed to know which of these meanings applies to it? I suppose by looking at the cards surrounding it and using intuition, but if anyone can suggest anything more practical, methodical or pragmatic, I'd be interested to here it.
Pragmatically, whatever comes out of your mouth (or head) first. I'm the Queen of Pragmatism, me.