In the viewport ? Check "Thick" in unwrap modifier/configure
In the editor ? It should be fluo green already. You can chose the color in options>preferences of the unwrap window. Note that the edges color in the unwrap is the wireframe color of your object, so if it's black or gray, you'll have hard time to see them.
I meant in the viewport. Some times, even with thick seams, when arc rotating around the object, the seams gets thinner and thinner until I cant tell where the seams are. That I can deal with. The biggest problem I should have mentioned is when I select edges. If I'm in shaded with wireframe and select edges, it's almost impossible to see what I've selected.
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In the editor ? It should be fluo green already. You can chose the color in options>preferences of the unwrap window. Note that the edges color in the unwrap is the wireframe color of your object, so if it's black or gray, you'll have hard time to see them.