What is the best way to look at modern pennies for errors? Do you hold them at a perpendicular reading distance, with no magnification and normal lighting? When choosing pennies to look at do you choose only the shiny?
How modern are we talking? Shield cents? If you mean shield cents, I unwrap and toss them into the dump pile as quick as possible!
It has nothing to do with how the coin is held or what kind of magnification to use.. It's pure knowledge and experience of what to look for. IMHO
Read up on what kind of errors exist. Get to know the difference between damage and errors and use plenty of magnification to rule out near-misses. I use a 10X loupe.
Whatever means nessesary to see the coin. Whatever light, magnification, distance or angle it takes to see it. Know what you're looking for and looking at regarding errors. You can start here.... https://en.wikipedia.org/wiki/Mint-made_errors
This plus learn as much about the minting process as possible so you will be able to recognize if something could or could not have happened during the minting process.
For me it's harder to imagine just how the minting process works without some visuals. This is one time where doing a search on YouTube for minting coins really does help!
Modern to me is coins commonly found in change. I was looking at some pennies and imagined seeing "something" but realized I was almost cross eyed at that point. Seemed like looking straight on (perpendicular) with light from the rear might have been the best look.