There is also an audio cue that plays at the same time. Try casting a somewhat quiet spell with frag slotted and keep an eye on your hands and an ear out for the sound. Once you get used to it you should have a much easier time noticing. Also the skill icon will flash and have an animated boarder while it is proc'ed.
When you got a crappy quality TV like me you can't really see it but like other people have said... your hands glow purplish and if you listen carefully it kind of sounds like a sword unsheathing noise if that makes sense... Because the quality of my TV the only other problem I have is that 99% of skills for sorcs or any type of buff makes my god damn arms glow... So yeah it makes it even damn harder..