Suppose you have a small rose plant growing in a pot. How would you demonstrate transpiration in it?

Take a medium-sized small well-watered potted rose plant and cover it with a transparent polythene bag. Tie its mouth around the base of the stem. Leave the plant in sunlight for an hour or two.
Drops of water will soon appear on the inner side of the bag due to the saturation of water vapour given out by the leaves (the water vapours condense only if the outside temperature is cool enough.)
A similar empty polythene bag with its mouth tied and kept in sunlight will show no drops of water. This is the control to show that plants transpire water in the form of water. If tested with dry cobalt chloride paper, the drops will be confirmed as water only.