Is it possible to debug the ContentBlocks parsing?

ContentBlocks does its own parsing of a resource when the page is saved. That processes field and layout templates. The caveats of that detailed here, but to summarise: all content that is guaranteed to stay the same until the next save (i.e. the content entered on the page) is processed on save, while things that may change through other actions (e.g. chunks or snippet calls) are left in-tact as tags in the content. 

The result of that parsing is saved to the resource content, which MODX will parse further when the page is rendered.

If you're having issues parsing a certain complex page, or perhaps you just want to know what ContentBlocks makes of a page before it is completed by the MODX parser, there's a simple trick to see the result of ContentBlocks' parsing. Right click the resource in the tree and use Quick Update to look at the content there. Note that any edits made to the content in quick update will be overwritten.