How can I know which asset it is when I select it in foliage mode in the viewport?
Hey @LegendaryLapuras, welcome to the Unreal community!
Currently there is no way to tell exactly which type of static mesh you currently have selected while in editor. A workaround for this is to perform a line trace and print the resulting component hit; this will return the name of the static mesh.
Selecting in foliage mode allows you to rotate, scale, or transform the foliage. Selecting foliage in Select Mode selects the instanced foliage actor, which will more than likely contain multiple foliage meshes. While the foliage mesh you want will be on the list of components, the currently selected component will not be highlighted so a line trace is likely to be the most direct option.
I hope this information helps!