Yes, I can definitely tell the difference though not in a skinny flat white from a coffee shop.
The difference is quite easy to tell as it is just a different taste.
My son was telling me he would not drink my milk -hi lo as it is not as nice and all the goodness has been taken out of it to make it skim milk.
OMG yes!. You can tell just by looking. Skim milk looks almost translucent by comparison. You can see how watery it looks when it's next to 'real' milk. And don't even start me on the taste difference!