Testing a variable

Here's a rule:

{
        :0  cb
        REALFROM=| $BINDIR/formail -X'From:'
        :0  fh
        | $BINDIR/formail -i"$REALFROM"
}

What I need to do is test REALFROM after it is generated. 
Basically, if REALFROM="" I don't want to run the second formail
command.   What's the syntax to test REALFROM?

Sincerely,

Brian Rankin
Far West Laboratory for Educational Research & Development