Check the event listing. Some event organisers will add the dress code of an event onto the event listing.
Check the event's social media posts. Some event organisers will add the dress code of an event onto their social media posts.
Check the venue's website or social media. Some venues may list their dress code requirements on their listing.
Please contact the event organiser directly if you are unable to locate the dress code information in these two locations. You can find a guide on how to do this here.