Identifying conflicting extensions in classic Mac OS
I know exactly what you're talking about. It's called InformINIT and it was one of those tools that many of us Mac experts used to narrow down problems with. Even after Apple's Extensions Manager was released with Mac OS 7.5, InformINIT remained an extremely useful tool to identify those pesky control panels and extensions!

In summary:


Yes, you can still download it from here
Yes, it works within Sheepsaver!


As an aside, don't forget you can ask questions here about your Sheepsaver problems.