Why can't you just give it the full filename? Use
fullFileName = fullfile(folder, baseFilename);
I mean, you already have the folder because you were going to use that to try to figure out if it was on the search path. But instead of doing that, just construct the full filename like I showed you above.
Alternatively, just try to open the file and check if the file ID indicates it didn't find it.
fid = fopen(baseFileName);
if fid == -1 ...