At constant altitude
At constant airspeed
At recommended cruise power setting
With wings parallel to the horizon
Its nose is above the horizon
Its nose is above cruise attitude
It's flying at a high angle of attack
It is turning left
It is yawing to the left
The pilot initiated a roll to the left
The wings are inclined with respect to the horizon (left wing is down)
By dividing the windshields into sections and spending a couple of seconds scanning each section before moving on to the next one
By using wide, sweeping eye movements to make sure you are always know what's going on all around you
By listening carefully to the radio transmissions by air traffic controllers and other pilots, and making accurate and frequent position reports yourself
You don't. The instructor always has control, so you didn't have control to begin with.
Let go of the controls, so it's apparent you're no longer physically flying the plane and tell the instructor "You have control."
Say to your instructor "You have control" and let go of the yoke and pedals, so the instructor can move them freely.
Say "You have control" and wait for the instructor to respond "I have control" before letting go of controls.
Here's an interesting quiz for you.