How to check the variable is empty with spacing

How to check the variable is empty or not?

aaa=" "

how to check aaa variable is empty or just spacing?
If only spacing inside.. it will asume it is empty.
some are 6 spacing, or 8 spacing.. as long as variable is empty with spacing..

anyone can help me?

From man test

[ -z $var ]

Oops! I was bit late.