Run the lookup
Go to Beckett's Graded Card Lookup, choose the grading line printed on the label (BGS for most modern cards, BVG for vintage, BCCG for its collector grade) and enter the serial number.
Beckett says sellers can link directly to the lookup from eBay listings. If a seller won't share a readable photo of the label, treat that as a warning sign.
Compare record to slab
Check that the player or character, set, year, card number and grade match the label exactly. With BGS, also compare the subgrades if the label shows them.
A real serial number can be copied onto a fake label, so a matching number is the start of the check, not the end.
Check the holder
Look at the edges and seam for cracks, glue or a case that flexes. Compare the label design with Beckett's current and past label styles, which it changed recently with new security features.
For an expensive card, ask for photos in good light, or buy through a platform that holds the card in a vault.
For repack pulls
When a repack platform shows a graded pull, run the same lookup before accepting a buyback or listing it. The cert confirms the card's grade; it says nothing about whether the pack price was fair.