How can a scientist assess whether a pure niobium (nb) sample is responsible for contaminating the lab with radioactivity?
test the niobium sample to see whether it now contains other elements.
test the niobium sample for the presence of niobium oxide compounds.
heat the niobium, and see if the level of radioactivity in the lab increases.
place the niobium under pressure, and see if the level of radioactivity in the lab increases.

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.