Two kinds
A fused sentence has no punctuation at the join. A comma splice has only a comma. Both are run-ons.
Finding them
Look for a second subject + verb pair after the first. Common trigger words at the join: it, this, then, however, I, we, they.
Frequently asked
Is a long sentence a run-on?
No. A run-on is about how clauses are joined, not length.
Is "then" a conjunction?
Not in formal writing. "I finished, then I left" is a comma splice; write "and then" or use a semicolon.