To ascertain whether a transverse section of a young stem is a monocot stem or a dicot stem, I would observe the following anatomical features under the microscope:

First, I would examine the arrangement of vascular bundles. In a monocot stem, the vascular bundles are scattered throughout the ground tissue. In contrast, dicot stems show a different arrangement of vascular bundles. Monocotyledonous and dicotyledonous plants show marked variation in the type, number and location of vascular bundles.

Second, I would observe the hypodermis layer. A monocot stem has a sclerenchymatous hypodermis, whereas dicot stems differ in this feature. The monocot stem also shows a large, conspicuous parenchymatous ground tissue.

Third, I would check for the presence of sclerenchymatous bundle sheath around each vascular bundle, which is a characteristic feature of monocot stems. The vascular bundles in monocot stem are conjoint and closed. Also, the peripheral vascular bundles are generally smaller than the centrally located ones.

Fourth, I would look for phloem parenchyma, which is absent in monocot stems. Additionally, water-containing cavities are present within the vascular bundles of monocot stems.

Finally, I would consider the potential for secondary growth. Most dicot stems undergo secondary growth, whereas monocot stems do not exhibit secondary growth.