Base Set and Wizards of the Coast sets (1999-2003)
Base Set, Jungle, Fossil, Team Rocket, Neo. The value gap between a raw copy and a high-grade slab is the largest in all of TCG. Centering is the number one limiting factor.
Vintage Pokémon is the most demanding segment in grading: a blue back that shows every mark, irregular cuts from the Wizards of the Coast era, holos that scratch on contact. A pre-analysis keeps you from submitting a card that comes back an 8.
In short
To estimate a Pokémon card's grade before submitting it, photograph the front and back flat under diffuse light, then assess four criteria: centering (PSA requires 55/45 to 60/40 on the front for a 10), the condition of all four corners, edge regularity and surface condition. On vintage Pokémon, the blue back and centering are the two most common reasons a card falls short.
The pieces collectors send in first — and what to watch on each.

Charizard
Base Set · 4/102 · 1999
Centering and back whitening

Umbreon VMAX
Evolving Skies · 215/203 · 2021
Textured alt art, scratches easily

Lugia
Neo Genesis · 9/111 · 2000
Frequently miscut

Rayquaza VMAX
Evolving Skies · 218/203 · 2021
Black borders are unforgiving
The defects that cap the grade, in order of frequency.
1999-2003 print runs are notorious for loose cutting. A 65/35 front is enough to cap a card at 8, however perfect it is otherwise.
The Pokémon back shows the slightest corner or edge whitening. It is often what separates a 9 from a 10.
The holographic layer on Charizard, Blastoise and friends scratches from simple rubbing in an unprotected binder. Raking light reveals them.
On modern sets (Sword & Shield, Scarlet & Violet), dark borders show white micro-chips straight out of the pack.
Two segments deserve their own method.
Where grading changes value the most.
Base Set, Jungle, Fossil, Team Rocket, Neo. The value gap between a raw copy and a high-grade slab is the largest in all of TCG. Centering is the number one limiting factor.
Illustration Rare, Special Illustration Rare, event promos. High demand and limited print runs: grading secures authenticity as much as condition.
More consistent cuts and generally better centering than western prints, which statistically yields more high grades. Watch for company-specific tolerances on Japanese cards.
High volume, low unit value: only high grades justify the grading fee. This is exactly where a pre-estimate stops you losing money.
Two photos are enough to know whether the submission is worth it.