Apparently even expert ornithologists can sometimes have difficult telling the gender of a particular bird, especially if watching from a distance.
But there is an extraordinary method of keen observation which will soon resolve the issue, especially where a male and female bird of the same species are standing close together. Even the ordinary Joe Soap can have an educated guess based on observation.
Yes, even you can probably tell quite easily just by having a good look.
Give it a try. Have a look at the photo below. Can you guess which of these two seagulls is the female?