Do 3.5.0 Mods work with 3.0.7?
 

I know this is a really stupid question and the answer is probably "no the templates are different" but I just read a hack that said it was for 3.5.0 and claimed to work with 3.0.7 so for that matter I'm asking do 3.5.0 mods work with 3.0.7 or not?

Depends on how it's implemented. If it uses the plugin/product system, no, it won't work. If it's a straight hack, then it may work. Depends on the mod and where the changes are made.

And how exactly could one tell by looking at the code if it uses the plugin/product system?

Assume no unless told otherwise.

If it comes with XML Files => It uses the Plugin System.

so, kirby, you're saying if it doesn't use xml files then it will work with 3.0.7?

No. But it doesn't use the Plugin System then.

What mod exists for 3.5 that doesn't exist for 3.0.7? Most have been ports at this stage.

I think a good few 3.0.x Template Modifications should work for 3.5.x, though not all of them.

The majority of Code Modifications won't work, because core functions have changed.