Is there any way to find out if an assembly has been compiled with the TRACE or DEBUG flag set without modifying the assembly?
The only best way to do is check the compiled assemblies itself. There is this very useful tool called '.NET Assembly Information' found here by Rotem Bloom. After you install this it asociates .dll files to open with itself. After installing you can just double-click on the Assembly to open with it and it will give you the assembly details as displayed in the screenshop below. There you can identify if it's debug compiled or not.
alt text http://ruchitsurati.net/myfiles/asm_info.jpg
(source: ruchitsurati.net)