What you're looking for is a quick, bright sheen of light. A low noise as well.

You may have noticed already that in the moves list, it displays some attack combos that have "pause" as one of the actions listed in its sequence. What that means is you have to refrain from letting your itchy trigger finger click that attack button for less than a moment. I know that's a difficult task in itself, but you'll get there. Also, if you're going to start the combo on one weapon then end on a different one, that light will also help you when transitioning from one to the other.

If you're able to keep yourself from clicking for long enough, you'll notice a glint of white light on your weapon accompanied by a soft metallic noise. This is basically the games way of telling you that the 'pause' sequence of the combo is over, and now you can finish it off with the last few clicks.
Example 1[i.imgur.com](because Steam refuses to let me upload decent gifs)
Example 2[i.imgur.com]
Example 3[i.imgur.com]

P.S: You can also use this form of timing for gun combos as well.[i.imgur.com]

..and that's pretty much it! I learnt this trick not too long ago and thought it was a very neat little touch added by the developers. This is going to help you tremendously if you're starting to try out different combos. Once you learn the timing off by heart, you'll be perform them without having to constantly glance at your weapon for confirmation. I hope this guide has been of help to you, and may your button-mashing adventures be slightly more organized :)