Seems like fairly arbitrary specs for what is or isn't a number. Ah well. Don't expect this to work for scientific notation, complex numbers, or anything else that isn't explicitly tested here.

printf "%s is %sa number.\n", $_, /^-?\d+(?:\.\d+)?$/ ? '' : 'NOT ' 
  for qw/4 -7 0.656 -67.35555 5. 56F .32 -.04/;
[download]

4 is a number.
-7 is a number.
0.656 is a number.
-67.35555 is a number.
5. is NOT a number.
56F is NOT a number.
.32 is NOT a number.
-.04 is NOT a number.