I guess by “placeholders” you mean links to pages that don’t exist yet?
I don’t know of any way to search for unresolved links using core Obisidian.
However, there’s a plugin that does it very well. You can find it in the Community Plugins section called “Find unlinked files and unresolved links”, or on GitHub at Vinzent03/find-unlinked-files. I use the plugin regularly and find it very helpful.