it is possible to search for observations by observation field. the easiest way to start off is to open up an observation that uses the observation field of interest and then click on the observation field name. from there, you should get a menu that provides you with 2 different ways to find related observations:
for example, if you were to click on View > Observations with this field, you would be directed to this version of the Explore page: https://www.inaturalist.org/observations?verifiable=any&place_id=any&field:DNA%20Barcode%20COI. from there, you can add or change filters as needed to further refine the result.
i think this just depends on the context. ideally, i think you would weigh the entire body of evidence, including photos, description, location, etc., along with the DNA barcode. for example, if someone has a DNA barcode for a fly and IDs it as a fly, but the photo is a horse, you might want to ask a clarifying question to the observer in that case.