I am Ashlynn
It is not just about beers, it is about many alcoholic beverages.
About beer - Isinglass, gelatin, glycerin, casein are considered vegan unfriendly ingredients which you can find in many beers. As well craft beers might have some hidden unusual ingredients like honey or what ever.
Heineken and Corona are for sure well known and vegan! And Guinness Is vegan, I think they have all of their beer now in this way.
Nowadays I think it is important to be ready to serve the clients needs and at least to know one vegan beer in the bar you work. In another hand if you are not interested in argument you can just Google yourself which beer is vegan and order, or if you are vegan and drink I don't believe you don't know which beer is vegan.