exist(obj.struct.var) always returns 0 regardless if the var exists or not.

Answer:

If you're not sure if the obj variable is a struct or has a field named struct, you can call isfield twice.

y = isfield(obj, 'struct') && isfield(obj.struct, 'var');

The first isfield call will return false if obj is not a struct or is a struct that does not have a field named struct. Only if the first isfield call returns true will the short-circuit && operator cause the second isfield call to be evaluated. In fact, that documentation page includes an example (Change Structure Field Value) that is somewhat related to this question.

obj = 42; % Not a struct
y1 = isfield(obj, 'struct') && isfield(obj.struct, 'var') % false

obj = struct('abc', 'def'); % struct but without the correct field
y2 = isfield(obj, 'struct') && isfield(obj.struct, 'var') % false

obj = struct('struct', struct('var', 'std'));
y3 = isfield(obj, 'struct') && isfield(obj.struct, 'var') % true