Depending upon the build date you can generally tell if it's a 1.4 or 1.6. After April 2004
Serial number format: LNNNNNN YWWFF
L = Production Line # in this factory
NNNNNN = number of Xbox on that line produced this week
Y = Last digit of year (200Y)
WW = week of the year produced
FF = Factory (02 - Mexico, 03 - Hungary, 05 - China, 06 - Taiwan)
1.4/1.5 production starts in 2003 Week 30 til production of 1.6's started.
1.6's production started in 2004 Week 20 onward till end of production in 2005.
LNNNNNN 33WFF
1.4, 1.5
LNNNNNN 42WFF
1.6
You can also look for the presence or absence of the motherboard mounting post below the serial number sticker:
See https://imgur.com/a/HM6PU