you would have to check by going all the way up the family tree.. usually you will find relatives not far up.. it's one reason you hear and see people trying to do preservation grows and open pollinations on some strains that are still somewhat not poly-ed up and why many people are starting to gravitate to heirlooms and land races when they want some vigor
they (mendo breath and grateful breath) share a parent..
cannabis breeding is not this black and white
^^^
you can't make a true F1 with combining two poly's or two hybrids for the most part.. I'm not going to call myself an expert or a breeder but I have my goals..
A true F1 is made with two genetically stable and unrelated plants (all the way up the family tree). typically the entire point is to bring something to the table the other doesn't have and is stable in the plant with it so it passes onto the progeny. most people just think an F1 is any new strain created from crossing two strains but that would be the lazy mans understanding of it (no offense to anyone).. a random x is just a hybrid or poly-hybrid most of the time nowadays. true f1's typically come stable due to both parents being stable lines (ibl in some cases).. this is why breeders often have to take them to F2 to "open up" the gene pool when they are searching for specific traits for a line.. but I mean, who works lines anymore? so.. yea..
when you find a true F1 and experience real hybrid vigor it's astonishing..
polyhybrid's are much less stable and predictable, as the "F1" does not have stable genetics usually. Polyhybrids are great for experimentation, as the genetic diversity is broad and they can exhibit unexpected and desirable traits.