A lot of people like to use the term 'bitter' when describing a burnt taste in their espresso.  For me this unnecessarily complicates the issue.  For me 'burnt' is such a distinct and easily identifiable taste, even to someone completely new to espresso, that you should simply call it 'burnt'.

Note that a coffee can taste burnt for two reasons, (i) the roaster over did things and/or (ii) the water was too hot at the time of extraction.

Using this approach the word 'bitter' is then freed up to be used exclusively for describing over-extraction.  You might be thinking to yourself, how on earth will I know what over-extraction tastes like.  Trust me, you'll know.  It is a very distinct taste and I think can be accurately described as bitter, to the point of being astringent.  Mouth puckering if you like, as this will happen to you almost involuntarily when you hit over-extracted coffee.

The obvious early warning signal for over-extraction is an extraction time anything much over 30 seconds from the time the espresso leaves the portafilter.  The crema is also likely to be brown with a white spot on it.  In freshly roasted espresso the crema will become strangely foamy and dimpled like the top of the apple and lime fruit smoothies at Wagamamas, if you'll excuse the use of a curious simile.