If your specimen is a:and has stem/​leaf ar­range­ment:and the leaves have/are:and the bark is/has:and the twigs:and the fruit (if present):then you have:
tree╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood  genus Cornus
sub-opposite small ▲▲ teeth dark, lots of light-colored lenticels

inner bark=orange (scrape with knife)
often have a terminal thorn are small, black, taste disgusting after a few seconds—spit it out lookalike common buckthorn  Rhamnus cathartica
╟ alternate ___ smooth or
very small ▲▲ teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn;
lookalike glossy buckthorn  Frangula alnus
▲▲ teeth, visible within arm's length dark, lots of light-colored lenticels

if mature, looks covered with burnt potato chips/​crisps
are small, black, taste like 🍒︎ cherry even if sour lookalike ◼︎🍒︎ black cherry  Prunus serotina
no more than waist-high (<1 m) (<3 ft)▲▲ teeth no hair this plant, please continue below …

Verify with Google images.

Learn more about alderleaf buckthorn Rhamnus alnifolia

Discover Life Encyclopedia of Life Google Google images Michigan Flora Minnesota Wildflowers USDA PLANTS db Wikipedia