GWBasic problem in finding presence or absence of files

Sorry to be so out of date but I am upgrading and compling (Quick64) some GWbasic programs to run on 64bit machines. I need to find the presence or absence of a file. In a batch file it would program something like this ..., but I want this to execute within GWBasic.

If exist c:\progA then goto A else B
:A do something
:B do something else

Thanks for any help