Which phase of the writing process involves checking the content to see if the essay is sufficiently detailed and adequately supports the thesis statement?

Writing  process has different  stages, the first four processes are the: prewritng, drafting, revising, and editing.

On the prewriting stage, it involves selecting and gathering ideas, creating lists, brainstorming, researching, reading  about writing styles of authors to discover more, gathering or collecting clips from other texts, and free writing. Teachers can be of great help to their student to do this stage. Developing an idea and connecting happens on drafting stage. Students begin to write on the drafting phase. Revising phase is when re-seeing and re-writing happens.  This stage emphasizes on the examination of the sentence structure, voice, word choice and the whole organization of piece.  The checking for spelling, grammar, usage, and correcting punctuation marks  is done in the editing phase. Editing phase involves the assessment for style and convention of the whole write-up.
Revising and editing are two different phases. Revising and editing are sometimes associated with each other but they are different.
Global issues are focused on the Revising phase such as content, organization, and style, while local issues like  grammar, punctuation, spelling, and formatting are examined on the editing phase.

Base on the above's description of the different stages of the writing process, the answer would be revising phase.