I did not, do ice cream trucks usually have one? If they do why isn't it posted? That's definitely possible, but can't be assumed.
If they are selling packaged products then just check the package.
However he was serving from machines so I would think they need to be certified (and certainly for CY) .... it may not have been posted on the outside of truck but if an agency was certifying them, they'd have the letter somewhere.
Make sure to confirm exp date, company name and address