The gemara is mesupak whether standard simanin are d'oraysa or d'rabonon. But the gemara is clear that siman muvhak, and tevias ayin are definitely d'oraysa and would be sufficient proof to return a gett to the one who lost it. Siman muvhak, rashi explains "דאין עדות ברורה מזו" - meaning that it is conclusive evidence beyond the shadow of a doubt. It would seem that DNA would qualify as siman muvhak when applicable.
Regarding tevias ayin, which is the person claiming to recognize it even though he has no specific simanim, Tosafos explains that although everyone is assumed to have tevias ayin, we only trust a Talmid Chacham. The rationale is as rashi and tosafos both explain, the talmid chacham won't lie. This is not to say that others will lie, rather others will use lower standards and claim to have a recognition of the gett even if they don't really have a recognition of the gett, but a Talmid Chacham who claims to have that recognition is believed to have it.
Someone asked this morning in my shiur, what is the torah source to trust tevias ayin? I answered based on Tosafos Rid, that the entire concept of trusting testimony of eidim assumes that their tevias ayin is sufficient. The Tosafos Rid writes:
ואי אמרי עדים שאנו מכירין אותו בטביעות עין בלא שום סימן כשר, שכל עדות העכדים בעלמא בטביעות עין הוא כגון הא דאמרי כלי זה שביד ראובן של שמעון הוא או פלוני הרג את הנפש וכל כיוצ"ב, היאך מעידין כי אם בטביעת עין
Perhaps the fact that the Torah believes eidim, confirms that the torah considers tevias ayin to be valid. Just that with an eid echad, there is a chashash of lying, unless he is a talmid chacham. When he is a talmid chacham, his tevias ayin will still not be sufficient in court when 2 eidim are required, but for a lost object or to eid echad ne'eman b'issurin, tevias ayin works.