>If GM crops are capable of being wind blown how does his activity differ at all from someone trying to naturally select for crops with herbicide resistance?
The only "naturally selected" herbicide resistant crops were made in a lab using fast-neutron bombardment. Not exactly natural, or amenable to traditional methods.
If you tried to select for herbicide resistance in a normal field, all you would get is weeds and a lot of dead crops. It is not impossible to do, simply infeasible.
>Given enough time a superior GMO plant could take over a field all by itself with no intervention
Then you sue the creator and make millions. There is a long precedent with inseticides/herbicide use.
Often if a pesticide blows on the wind into a neighboring field and causes damage, the sprayer is liable. This has already been used to litigate geneflow from one field to another.
>A related problem is that it's difficult/impossible to tell if you're growing a patented seed without expensive tests.
Sampling your soil/crop is actually quite routine in farming. A PCR for a trait can be easily and cheaply performed.
Though, this shouldn't be necessary because farmers usually keep tight control of their genetics. Most commercial farmers (conventional and organic) use hybrid seed. This seed is most fit only for one generation, and as a result they order seed from seed companies to maximize yield. As a result, gene flow is not a huge problem.
Obviously it might be more of a concern for heirloom seed providers, but simply being aware of neighbors and pollination distances can alleviate concerns.