How does one determine accuracy of a watch with no seconds hand?

This piece of mine only has a running chrono seconds hand (the thin white one) and an elapsed chrono minute hand (the black one with the orange B727-200 on the tip).

How do I set the time accurately to my smartphone reference? And how do I determine precision over 24 hours? Don't ask me to buy one of those silly TimeGrapher machines or download some phone app 'cause I ain't gonna do it.
Disappointed.