How would one go best about checking for existence of all files before building?

Let me explain; I mostly build stuff from the command prompt. No problems there, just put the build command and all in one .bat /.cmd file, and run it. It works fine. 

But, for the normal running of my program, for example, I need several source files for the build, and then an additional few data files, measured data and such.

Is there a way to test via a batch file whether a file exists, and if it exists just write OK?

file1.for OK  

file2.for OK  

datafile.txt OK
data.dat MISSING FROM DIRECTORY

How could this be accomplished?