Delphi compiler: how to find used units

I have a strange problem: I generate a new DXE2-Project and add a single unit to it, which I used in another project. Compiling the project opens another unit with an error. My problem: I can not find out whit this second unit is compiled. I can not find it in any uses-section. Is there a way that shows me the tree with the units (sorted by the way the compiler compiles them)? So I could find that unit in the right uses-section.

Thanks, Peter