SN-95 visual year identification guide
How to quickly tell the model year of a SN-95 Mustang

 
1994 was a complete redesign for the Mustang.  The square fox body was replaced by the smooth lines of the SN-95.  
1994 and 1995 Mustangs have the old tail light design with 3 thin horizontal lights.
1994 Mustangs can be distinguished from the 95s by the body piece just behind the front wheel.  This piece on a majority of the 94s went from the door to the wheel well and did not turn up.  Late in the model year it was replaced by the newer style which turned up and goes 8 inches up the wheel well.
The spoiler on the 1994 and 1995 Cobra was different from the GT's.  The Cobra shared the same hood as the v6 and GT.  The Cobra said "Mustang" across the rear bumper.
1994 and 1995 Mustangs have the old tail light design with 3 thin horizontal lights.
1995 Mustangs can be distinguished from the 94s by the body piece just behind the front wheel.  This piece on a majority of the 94s went from the door to the wheel well and did not turn up.  All 95s have the newer style which turned up and goes 8 inches up the wheel well.
The major change in 1996 was the introduction of the Modular engine.  You can't see the engine from the outside of course, so the easiest way to tell a 96+ Mustang is the new tail lights.  The 96+ Mustang has 3 Vertical tail lights.  The GT badge on the side of the 96-98 says "GT 4.6 liter" instead of "GT Mustang".  The 1996 Mustang can be distinguished from other years because it has an egg crate type filler in the grill.
The spoiler on the 1996 - 1998 Cobra was now the same as the GT's.  The Cobra switched to the new hood with 'nostrils'.  The Cobra said "Cobra" across the rear bumper.
The 1997 Mustang looks very similar to the 1996.  The only visual change was that it no longer had the egg crate type filler in the grill.
The 1998 Mustang was also very similar to the previous year.  The major visual change was on the inside.  The pod clock is no longer on the dashboard, the clock is now integrated into the radio.  Instead of an ash tray next to the emergency brake there are now 2 cup holders.
1999 was a major change to the body style of the Mustang incorporating Ford's "new edge" design.  The 1999 Mustang can be distinguished by the circular 35th anniversary badges on the sides.  The GT badge was on the left side of the trunk lid.
The 2000 Mustang looks very much like the previous year.  The round side badges have been replaced by the familiar running horse, 'GT' and Cobra emblems.
2001 Mustangs now have tinted headlight lenses like the Cobra R. The spoiler is changed back to the 94-98 style connected to the trunk lid by posts instead of at the edges like the 99-00 spoilers. The radio and cd player are now one big unit instead of two seperate components.