Dan, I think your question contains your answer. I know how my wife and I are addressing it.
Until you are pretty confident in your ability to identify everything in your seed mix at pretty much every stage of its growth, you don't plant randomly;)
We did just that last year with her herb garden - planted randomly - and ran straight into the simple fact - we did not know
enough to be able to recognize which was which when things started germinating. The spot had serious
volunteer issues, so we were not only trying to tell basil from sage from dill, etc. but also facing a bunch of random unknowns thrown into the mix along with what we had planted.
Result was a pretty thorough failure of an herb garden, with the dill eventually coming through as the only recognizable successful herb, and some poppies. The rest were either weeds or something we had planted but failed to recognize (embarrassing, but educational)
So, come next spring, we plant herbs again, but this time in discrete patches. Then we're only dealing with recognizing One intended plant in a given area, and anything that is not that plant we can beat back, as we choose.
Part of the equation is always staying within the limits of the practitioner's skill set.
Now, I do have questions about how one harvests some of the incredibly complex plantings that turn up in
permaculture, but that has less to do with identifying what is what than with inefficiencies inherent in having, for example, your tomatoes spread about amongst your beans, squashes, potatoes, etc. If all of plant X is in one small area, when it comes to harvest time, it's all there and you can collect it pretty efficiently.
But, if it's distributed amongst other plants and over a somewhat broad area, then you have issues of going to each plant to be harvested, getting through surrounding plants not yet ready for harvest with minimal disturbance, and so forth.
Seems that part of this is a matter of knowing your plants and setting up your design so that you've got stuff organized in compatible fashion, maybe even in a successional manner, so early peas get harvested before the tomatoes beneath them are big enough to have a problem with it, and the peas are done and cut back before you need to get to the tomatoes..
More questions ;)