Checking if a file exists in VBDOS...

IF LEN(DIR$("PATH\TO\FILE.TXT")) <> 0 THEN
  PRINT "File exists!"
ELSE
  PRINT "File does not exist."
END IF

...or the other way around...

IF LEN(DIR$("PATH\TO\FILE.TXT")) = 0 THEN
  PRINT "File does not exist."
ELSE
  PRINT "File exists!"
END IF

And of course, you can replace the PRINT parts with whatever you want.
If you are wondering if this would work in QB45, then the answer is no.
QB45 does not have the DIR$ function. However, if your goal is to not
create a graphical program but still want to use something like QB45, 
you can still use VBDOS without creating any forms and you get extra
built-in functions with more RAM access.