awk: verify same number of columns

## prints number of columns if it changed from previous line

> awk -F"\t" 'NF != oldnf { print NF; oldnf = NF }' filename |head

|head added just in case file is very long

## using a script

> awk -f script.awk filename | head

script.awk:

BEGIN {
 FS="\t"
 oldnf = 1
}
{
  if(NF != oldnf) 
  {
 print NF
 oldnf = NF
  }
}

Other examples in 
http://sparky.rice.edu/~hartigan/awk.html