I can remember getting a sense, from previous eclipses, to look at the sun briefly with the protective glasses. Just a sense of not wanting to build up an image of the sun on my maculae for a duration of time. Manufacturers of these eclipse glasses state that the wearer can look at the sun through them for a while. I'd feel better about it if I had some sort of light transmission device that could be used to measure the UV and IR throughput of eclipse glasses. "Show me the evidence".
Of course a bigger threat is looking through telescope at the sun with an inadequate filter. Permanent damage can occur in less than a second.