Hidden Items

So is there any dowsing machine or something to find them or am I just suposed to smash C all over the world? If so, is there any list with all hidden items? Or for the tesseract items too? Wiki only shoes mega stones

There’s no dowsing machine or anything similar in Insurgence. There’s no specific page for hidden items in the wiki either. However, the individual wiki pages for routes, cities and various other locations have the items listed and it’s mentioned whether the items are hidden. For example:

Tesseract items don’t have a sperate page either but you could use this instead: https://wiki.p-insurgence.com/index.php?title=Special%3ASearch&profile=default&search=Tesseract&fulltext=Search