So, this might seem like a silly question, but is there any way to tell whether or not common vegetables grown from collected seeds have crossed before the fruiting stage? I mean, with tomatoes, even though they don't often cross, I'm not sure that I could tell even after the fruiting stage unless they ended up either the wrong colour or the wrong shape. I'm also fairly certain that it would be nearly impossible to tell pepper plants apart until they produced.

But I'm thinking more of, like, the brassicas, or squashes. Is there any easy way to tell them apart, say, from their young leaves? Especially if they've crossed?

Last year what ended up being a "wild" acorn squash that had clearly crossed with something else grew on the shore near my grandparents house and thought it was clearly a squash-like plant of some sort I puzzled for ages over what it would eventually produce. After the fruit began to come it was pretty clear that it was acorn squash and something (I'm still not sure what the something was, probably a zucchini). Regardless of the cross it ended up being quite edible.

But I digress. Does anyone know of an easy or effective way to tell if crossing has taken place before too much effort has been put into the plants? Does taste work with brassicas (since they should be completely edible)? Can you tell crossed squash plants apart by leaf shape?

I have some seeds collected from last year, and I've grown from collected seeds with success before, but this year I'd like to try collecting some others that are more of a challenge. I know the techniques for preventing crossing of various species, but I'm curious to know if there's any way to verify whether it was successful or not without actually growing out the resulting seeds to maturity.