I put this together a few weeks ago then forgot about it. We see a lot of these cars and in the past they've just been fairly robotically IDed as 2000-04/05 but there's a few obvious changes that happened during the run that can narrow down the year range. It's useful to keep this as a thread because the knowledge might be lost depending on who's active or looking at a specific time (see the issues with the other police favourite, the Crown Victoria, recently). Also, if any new knowledge is spotted it can be added to this guide.

Original Versions:
Base Impala/9C1:

1. Black sills/rocker panels.
2. Always with black trim on bumpers and side mouldings (unless painted over)
3. Steel wheels on 9C1
4. Plastic covers or multispoke wheels on Base.

Impala LS:

1. Painted sills/rocker panels
2. Front foglights
3. Usually with Five spoke alloy wheels of two different designs
4. Headrests with holes (Standard 2000-03, not always fitted 04-05)

Year Changes:
2000

1. New Impala introduced this year.
2. Two piece bumper trim with body coloured number plate surround. NOTE: some later models have been spotted with this.

2001

1. New bumper trim. One piece with plastic surround to number plate
2. Emergency trunk release added (well we might see that one time)

2002
Nothing found so far

2003
Sports Appearance Package now available on LS trim

Sports Appearance features:

1. Body coloured rear panel
   
2. Body kit with extended shirt, front spoiler etc
3. Slightly different 5 spoke rims to normal 2003 LS.

2004

1. Body coloured bumper trim and side protection strips on Impala LS (Base and 9C1 continue with black plastic trim.)
2. Rear panel is now black rather than tinted red
3. SS trim added to the range.

SS features:

1. Has features of Sports Appearance Package.
2. Specific 17” alloy rims
3. Unique spoiler which is more closed off than the one used on normal models
4. Only available in black.
   

2005

1. Chevrolet bowtie on front is now brass/gold rather than blue
2. Last MY for this generation of Impala
3. SS model now has headrests with holes.
4. New colours offered on the SS (blue, silver, red etc)

Pictures:
2000 bumper trim and 2000-03 rear panel:


2001+ bumper trim and -03 rear panel:


2003 LS with Sports Appearance Package:



2004-05 rear panel:


2004 LS showing the 2004-05 body coloured trim on that trim and the SS:



2005 SS showing the SS specific rear spoiler (Gold rear badging is just an add on):

... and the 2005 gold badge.


And if anyone spots new knowledge or corrections this post can be updated/correctly.