Use the nearly undocumented DOS command TRUENAME to display the true name of a file on the screen. For example the command
truename c:\os2\dll\..\install\..\..\config.sys
will print C:\CONFIG.SYS to the screen. ("nearly undocumented" because TRUENAME /? works but you can't find anything about it in the online help)
Source: The OS/2 WARP Survival Guide
In OS/2 REXX programs you may use the function STREAM with the third parameter set to QUERY EXISTS to achieve the same result for existing files.