Trademark: what should I check before filing?

Most trade mark problems start before filing: a mark that is too descriptive, a conflict nobody searched for, or an application in the wrong name.

Moderate complexity - several routes or missing facts

The exact mark (word, logo, or both) and the goods or services it will be used for.
Who will own it, and who created any logo.
When it was first used, if at all, and where.
Search results for similar marks.

The honest answer usually needs someone to look at your actual disclosure, your timeline and the prior art. That is a conversation, not an article.