Writing custom Snort rules and what to check if they are correct? ..up to a certain point.
Well dumbpig [1] by Leon Ward is what you are after. For a good example check out VRT Sourcefire's blog entry [2].

...while you are at it have a look at Snoge [3] "Take your Snort or Sourcefire IPS events and place them onto Google Earth.".

Links Used:
[1] - dumbpig - http://leonward.wordpress.com/dumbpig/
[2] - Syntax Checking your Snort Rules - http://vrt-sourcefire.blogspot.com/2009/08/syntax-checking-your-snort-rules.html
[3] - snoge - http://code.google.com/p/snoge/