You should always collect consent on every form by adding a consent confirmation checkbox to forms. This way you're gathering express consent from contacts who fill out the form.
The consent confirmation checkbox is automatically hidden for contacts with known express consent.
STEP 1: On the form editor, click the "Add new field" button
STEP 2: Select "Consent and subscriptions"
STEP 3: Set display format and defaults
STEP 4: click "Add"