args and test of passed paramters

Hi,
I would like to call a R script sometimes with:
?R --slave --vanilla --args Debug=1 N=25 < test.R
and sometimes with
?R --slave --vanilla Debug=1 < test.R
or similar.

Inside the R script I get the arguments with:

args=(commandArgs(TRUE))
if(length(args)==0){
?print("=== ? ?No arguments supplied. =============")
}else{
?for(i in 1:length(args)){
? ?eval(parse(text=args[[i]]))
?}
}

But I don't know, how can I test inside the script if the parameter N
was passed to script or not. I am looking for something like
is.defined(N), but I could not find it until now.

Any hint?

Juergen