merge folders with the same name

When attemtpting to merge folders with the same name, I am pressing and holding the option key whilst dragging the source over the object folder and releasing the folder when the object folder flashes / pulses ( the option key is still held). Once the merging processes has started I release the option key. PROBLEM: some folders may be empty, no particular reason why other than with time I have restrucured the folder structure, moved files and not cleaned-up after myself, however I have found that OS DOES NOT recognise that I am attempting to merge a full folder and an empty folder, and if I don't know one is empty I am at risk of over-writing a full folder with an empty one. Any logic or solution I can use to gain confidence in Finder and the merging process please?