The IS MISSING and IS NULL operators, which are used with a WHERE statement, can handle character or numeric variables. They also work with the NOT operator:

*** create a sample dataset with some missing values;
data makemissing;
    set sashelp.class;
    if substr(name,1,1) eq 'J' then initial='J';
    if 11 le age le 13 then newage=age;
run;
 
*** use missing;
proc print data=makemissing;
    where initial is missing and newage is not missing;
run;
 
*** or equivalently, use IS NULL;
proc print data=makemissing;
    where initial is null and newage is not null;
run;

This may be especially useful in a macro that takes mixed-type variables as parameters in a where clause.

Thanks to Mary Rosenbloom for submitting this tip on sasCommunity.org!