Simple WeakAura that tracks the number of swings you've done before the next Shadow Techniques proc.
The WeakAura provides an user option, to either show all the swings done (0-4) or to provide text (Soon, 50%, or 100%). Soon equals to the 2nd melee swing, 50% to the 3rd and 100% to the 4th.
The 3rd swing and the 50% markers are colored in Yellow, as the next swing could trigger Shadow Techniques.
The 4rd swing and the 100% markers are colored in Red, as the next swing will trigger Shadow Techniques.
This WeakAura doesn't use an 'update every frame', so it's performance friendly.
If the WeakAura isn't working, please update your WeakAuras addon. There have been reports of the WeakAura not working for people with old versions of the addon.