I don't know if there is a way you're supposed to be able to do it, but one thing you can do is right-click the image and pick Inspect from the context menu, which will open a panel with the HTML of the image highlighted, and part of that will be the alt text if there is any.

Also I have a Firefox extension that adds "Copy Alt text" to the context menu when you right-click an image. I've used that and pasted into a text editor, which usually works as well.

Both of those are inconvenient, but at least you can find it. But there should be a button on the image to view it easily like there is on Mastodon.