You cannot grade a card through a screen. You can get most of the way there by asking for three specific things.
The three requests
Tilt it under the light. Surface scratches and print lines only show at an angle. A flat-on photograph hides both.
All four corners, close. Softness at one corner is the commonest unmentioned flaw.
The back. Back centring is often worse than front and is frequently not shown. Ask every time.
What centring means
How evenly the image sits inside its border. Measured as a ratio, and a card that is visibly off-centre loses a great deal of value however clean it otherwise is.
You can judge this yourself on a held-up card better than you can from a photograph.
Graded cards
A graded card is sealed in a holder with a number — common scales run 1 to 10. The holder settles the condition question, so what is left to check is that the holder is intact and the label matches the card.
Ask for the label and the card in the same shot.
On trimmed cards and fakes
A trimmed card has been cut to improve its edges. It is the thing to worry about on a valuable raw card, and the reliable check is measurement against the standard size, which somebody has to actually do — not something you can settle from a blog post.
If a card's value rests on it being untrimmed, buy it graded, or buy from somebody who will stand behind it. The protection covers "not as described".
Questions people ask
What does "raw" mean?
Not professionally graded.
Should I only buy graded?
For high value, it removes the argument. For everything else it adds cost and wait.
What if a graded card arrives and the holder is cracked?
Raise it — damaged in transit is covered.
Can I ask the seller to grade it first?
You can ask. It takes weeks and costs money, so most will not.
More: condition words.