Starting from most obvious:
Check the history.
- Blank page?
- Blanked by author? [G7]
- Blanked by other? Revert
- In English?
- Exists on other wikipedia? [A2]
- Does not? [Move to draft or tag with Not English (debated on NPP talk page(?))]
- Understandable?
- Nonsensical? [G1]
- No context? [A1]
- Appropriateness of content?
- Attack page? [G10]
- Entirely promotional/spam? [G11]
- Vandalism/hoax? [G3]
- Made-up by creator? [A11]
- Topic already exists? [A10 or redirect]
- Copyvio? [G12]
- Claim of significance?
- Insignificant people, individual animals, organization/company, web content, organized event, band? [A7]
- Insignificant musical recording (for example, musical recording of a group which does not have an article)? [A9]
- Insignificant anything else? PROD/AfD
- Cleanup/tags
- BLP without sources? [BLPPROD or Move to draft]
- Other topics without sources? [Tag]
- Categories? [Tag]
- Stub? [Tag]
The extra mile (but not required):
- Stub sorting
- Adding to categories
- Classifying by wikiproject
- Copyedit
Things that are always considered notable as long as they do exist (or so I've been told in the past):
- Species of organisms
- Experimental drugs at phase II of clinical trials
- Genes
- Train stations
- Human settlements
- Mountains, rivers
- Listed buildings
- Provincial/state legislative districts[1]