Measure quantitative (Leaf length, internodal distance etc) and qualitative (presence of hairs on leaves, number of secondary veins) vegetative and reproductive characters and conduct a morphometric analysis.
Morphological traits (MT) are the physical form and external structure of plant species. The main MTs are shoot, root and reproductive systems, such as petal number, leaf shape, flower colour, etc. So you need to decide on the key characteristics that you like, measure them, then compare species :)