SYMEXIST to check if a Macro Variable Exists

%let a=1;

data _null_;
      if symexist('a') then put '**** a exists';
          Else put '**** a doesnt exist';
      if symexist('b') then put '**** b exists';
          Else put '**** b doesnt exist';
run;

**** a exists
**** b doesnt exist

NOTE: DATA statement used (Total process time):
real time 0.00 seconds
cpu time 0.00 seconds