If the data in the fields can be concatenated together (i.e. have the same number of rows/columns/...) then you could use a comma-separated list to create one array, and use that array. For example:
>> S(1).a = 1;
>> S(2).a = 2;
>> S(3).a = 3;
>> idx = [S.a]>2
idx =
0 0 1
>> S(idx).a
ans = 3
Clearly you can do that for any fields. If the data cannot be concatenated together, or have sizes that would be ambiguous when joined together, then you will have to use a loop, even if implicitly inside cellfun. For example:
>> cellfun(@(v)any(v(:)>2),{S.a})
ans =
0 0 1
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