Take another metre rule and place it vertically near the oscillating tip of the initial metre rule.
Note/Mark down the reading of the vertical metre rule.
Depress the rule and allow the mass to oscillate.
On the vertical rule, try noting/marking the maximum and minimum heights the mass reaches as fast as possible, without touching the oscillating mass.
Then, compare your results. If the maximum displacement upwards is equal to the maximum displacement downward, it's a free oscillation.