At first, I have loaded data using S = dir('*.mat');
The dir function does not load data. All it does is generate a list of files in the directory with the .mat extension. To load the data you'll need to use the load function. Then if you want to check if the loaded data contains a variable named B (that has been loaded into a field of the output of load you can use isfield.
data = load('census.mat')
doesItHaveAVariableB = isfield(data, 'B')
doesItHaveAVariableCdate = isfield(data, 'cdate')