the fastest way to test if variable is empty

Hi all.

Can anybody tell me the fastest way, how to test, if variable contains some 
text or number?

For now, i am using test "string-length($variable)=0", but seems not to be the 
fastest.

Somebody tells me to use test "$variable", but with my XalanC, it seems to be 
true everytime.

Thanks.

PS. variable can't contain nodes, it may contains only alphabetics or numbers 
and whitespaces.

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S pozdravom,
Dusan Zatkovsky