So basically there is no 100% sure way to tell if a seed is true to the parent...
A bit disappointing. :p
This is one of the biggest impediments to breeding.
I would agree that there is no 100% certain way to know (in the absence of genetic analysis), but there are a few strategies that can be used in intentional breeding to be very close to certain (maybe 97% sure). One would be for example if the seedlings display any obvious traits that can easily differentiate them from their seed parent, usually leaf shape.
It's a bit too complicated to get into here, but if you carefully select the varieties, using the right combination, and emasculating the male/female parts of the flowers of the two parents, you can be pretty sure.
And again, with certain citrus varieties you can be pretty sure. Oranges are almost certain to produce all nucellar seeds, while original species citrus like pomelo, citron, many varieties of mandarin, are sure to produce all zygotic seed.
This is very generalized but most of the seeds in lemons and grapefruit will be nucellar, while some will be zygotic.