Goes without saying. It’s rare to find GMO vegetables even commercially (only zucchini and potatoes even could be), let alone in a garden.
But that’s not what we’re talking about. We’re talking about ensuring the GMO vendors haven’t ever touched the seed. How are you ensuring provenance as to avoid the trust issues?
If you're trying to do a "reflections on trusting trust" reference, well, this is the real world. You might not ever be 100% sure that something is what it says it is (you might be a brain in a jar!), but it's usually better to get it from the guy you trust than the guy you don't.
Of course, at least within some margin of contamination. Identity preserved allows no more than 0.5% contamination, which is the best in the biz. But, the problem is that the earlier commenter didn't trust GMO vendors, not GMOs themselves. As in: Non-GMOs from a GMO vendor are equally untrustworthy. Hell, even a hat with a GMO vendor's logo on it is untrustworthy according to that commenter.
Unless you're sticking to food that doesn't have GMOs to begin with (which, to be fair, is almost all food!), you're going to be searching high and low to find a vendor that doesn't also dabble in GMOs.
If you buy commercial products, the non-GMO seeds are unquestionably being sourced from the very same GMO vendors, and thus can't be trusted either.