as someone with striking skills, i take immediate notice if someone is afraid of getting hit or is forced to stop momentarily to cover up or defend when punches are coming at him. this is the signal to move in. a good striker will be able to keep moving and working while punches are coming at him and he is also able to both defend and attack at the same time, he will not stop like the "deer caught in the headlights" nor will he cover up completely.
a good example is this recent fight between mike bisping and charles mccarthy. mccarthy is a grappler while bisping is a solid striker. here we can see mccarthy completely cover up when strikes were coming his way, allowing bisping to take advantage by grabbing him and delivering knees. mccarthy is a tough guy, but realistically how many strikes are you going to be able to withstand while covered up like that???
Michael Bisping Vs Charles McCarthy Fight Video UFC 83 | MMA TKO