If it is dye, it will be fluorescent. Aim a UV lamp at it and see if it glows. Now, just because it is dye doesn't mean that it definitely came from the AC system, but dye is routinely added there and not normally added anywhere else.

I realize how difficult it is to change the evaporator in that car. Sealer is used for situations like this, but mainly for an owner who does not plan to keep the car very long and thus doesn't care much how many more years of AC it has left, as long as it runs for a season or two. If this does not describe your relationship to your car, stop thinking about sealers.