How can you tell if a seed is only a cross and not a stabilized seed?

You won’t be able to look at a seed and visually identify any abnormality that would be obvious, however the way the package is labeled or what the strain is called may hold the answer.  If the strain is labeled, “ Sour D x OG”, then you know right away it is a cross or crossbred seed and it’s not stable.  If the name of the strain is Sour OG then it’s a good possibility that it is a cross and not a stable seed.  There can be exceptions such as a Sour OG that has been stabilized or Blue Dream which is NOT a stabilized plant or seed.  So even the name or packaging can be deceiving.

FYI:  Blue Dream is a phenotype cross between Blue Berry Kush and the famous Super Silver Haze.  There are currently three distinct different versions of Blue Dream: The Indica dominant light blue/green large sponge bud with a sweet fruity Blue Berry taste swirled with dank Haze. The Sativa dominant Blue Dream with long lime green solid flowers and with a taste that is more Haze than Blue Berry.  Finally there is the Sativa Dominant gold version, which has a milder taste,  long thin golden buds and is not as potent as the other two Blue Dream strains. Think Gorilla grow quality.