You can use the FOPEN to test. If FOPEN returns <0 you can show a message to explain the problem to access the file (user´s rights or other system problems). I use a OPEN routine (writed by me) to open each file and the FOPEN command is tried first. It´s fast and the result is good. Something like that:

LNhandle = fopen(xxxx)
if LNhandle<0
Error message
else
fclose(LNhandle)
endif
use &xxxx

I hope it helps you.