How do I know the cannon interaction worked?

The cannon changes from upright to tilted and a red burst appears above the obstacle area; use that visual state as the checkpoint.

Should I try to jump over the monster and cannon?

No. Use the jump to trigger the monster/cannon interaction, then cross on the floor after the cannon fires.

Mr Potato level guide

hard

Trigger the cannon beside the giant monster, wait for it to tilt and fire, then cross the cleared floor to the right-hand door.

Layout: The potato starts on the far-left side of one continuous floor. A giant monster blocks the middle, a wheeled cannon/cart sits immediately to its right, and a chained door waits at the far right.
Goal: Activate the cannon sequence so the monster no longer blocks the crossing, then enter the right-hand door.
Opening: Approach the monster from the left and use a jump at the monster/cannon interaction area instead of trying to walk through the obstruction.
Danger Zone: The first contact can knock the potato backward or make it tumble. Recover on the left and make the higher jump/interaction attempt rather than repeating a floor-level push.
Mechanics: A successful interaction changes the cannon from upright to tilted and produces a red burst above the obstacle area; that firing state signals that the right route is open.

Do not judge the first knockback as a failed level; use the left side to set up the second jump.
The tilted cannon is the confirmation to stop interacting and run right toward the door.
Think in chain clears. The best move is the one that sets up the next two moves, not just the quickest current match.

Start on the far left and walk toward the giant monster until the potato is just to its left.
Let the first contact resolve if it pushes or tumbles the potato backward, then regain the left-side position.
Jump again toward the monster/cannon interaction zone to trigger the cannon rather than trying to pass the monster at floor level.
Wait for the cannon to tilt and fire; the red burst above the obstacle area confirms the sequence.
Walk right across the cleared floor and enter the chained door.

Repeatedly walking into the monster at floor level does not open the route and can keep knocking the potato backward.
Running right before the cannon tilts and fires leaves the monster blocking the crossing.

How do I know the cannon interaction worked?
The cannon changes from upright to tilted and a red burst appears above the obstacle area; use that visual state as the checkpoint.
Should I try to jump over the monster and cannon?
No. Use the jump to trigger the monster/cannon interaction, then cross on the floor after the cannon fires.