There are two 'rules':
1. The shell must NOT be longer than the chamber. The shell length (mesured when open/shot) is either designaten in inch or in millimeter. Common US is 2 3/4 or 3", the European equivilents are 12/70 or 12/76. BUT, there are many older European 12/67, so better be safe than sorry. AND, there are even shorter chambers. If you ask 'what's the problem', if the shell is longer than the chamber, the shell extends into the barrell behind the chamber. As the barrel is of smaller diameter then the chamber, the shell partially clogs up the barrel and this results in high pressure.
Chamber length is written to the barrel, either in US or Eoropean style. This can mean there is just a '70' written somewhere.
2. There are 'Magnum' shells sold. I have some French 'Mirage clever' Buckshot which says '-70-' and 'max 1050 bar'. My Russian SXS says '12x70' and '650 MPa'. Well, the length is OK, both are 2 3/4" (70mm), but 65 MPa is 650 bar, so even if my Russian shotgun looks like it would also fire dynamite, firing 1050 bar rounds from a 650 bar shotgun doesn't sound right. BTW, bar and MPa are both European pressure units. So if the ammo or the box says anything like 'magnum' or 'xxxx bar' and you don't understand, ask someone who does.
As a shortcut, any 2 3/4 (12/70) shell in a 3" (12/76) should be ok as all 3" are 'magnum'.
If this sounds complicated, that's because it is!