I like that "slightest amount of stress will tend towards being male 50% of the time".
You will either get a male or female = 50%.
While I obviously cannot speak for others on here, I do not pull this stuff out of my ass. I also don't claim to be any kind of expert or professional, but my methodology is far from lost. You can get tons of reliable information about plants and soil sciences, in general, right online from various sources including academia. When it comes to cannabis-specific info things tend to get a
bit foggy, but there are actually some individuals in this industry (cannabis breeding) who are trained horticulturists, botanists, scientists, etc. and have been doing this long enough.
Saying that you can tell a female seed by visual inspection of any characteristics is far fetched at best; it is doubtful any professional breeders would make such a claim. However, sexual tendencies correlating to environmental factors (as well as genetics) really isn't so unimaginable. Most are already aware how a flowering plant under stress can begin to express hermaphroditism.
TGA Subcool Seeds comes to mind, I've read multiple times how TGA Subcool seeds under preferred conditions have 60\40 females to males or better. Granted, that at face value doesn't mean much and I have never grown any TGA seeds. But, I have grown Mandala seeds. Breeders, the good ones anyways, can give you valuable information about how they recommend their plants should be grown. The Mandala website has some great information including a grow guide detailing explicitly how the plants should be germinated and how they should not be germinated. Interestingly enough in the FAQ page it is noted that:
What is the male-female ratio?
The male-female ratio is generally around 50-60% in our strains. This is the normal result from at least one packet of seeds. But we do receive frequent reports that growers get up to 70-80% of females (which corresponds to our own experience as well). An unusually low female ratio can be traced to negative environmental factors. Overall there is a natural balance between the % of males and females among a bigger population of plants.
If you plant considerably less than 10 seeds, such as only 4-5 seeds, it is possible to get an unbalanced ratio: a disproportionate number of females or males. This has nothing to do with the genetics of the strain. Rather, it is a matter of chance. While one grower may get many females, another one has a high number of males. We have often received feedback from growers who had 4-5 females from 5 seeds, but in some cases these results were reversed and, instead, a grower may have many males.
There is a lot of good info on the Mandala website and they make damn good seeds.
http://www.mandalaseeds.com/FAQ