Giving here the keys from Reinstatement of the Sino-Himalayan species Senecio pentanthus (Asteraceae, Senecioneae) MING TANG, CHEN REN & QIN-ER YANG - Phytotaxa 292 (3): 253–261 (2017):

Key to Synotis pentantha and its similar species
1. Capitula conspicuously radiate ..........2

–   Capitula minutely radiate ..............3

2.   Leaves with prominent, distinct veins beneath ...........Syn. lushaensis

–   Leaves with obscure veins beneath .............Syn. vagans

3.   Corymbs dense, glomeruliform, 2–4 cm long ........4

–   Corymbs more lax, spreading, 4.5–10 cm long ....5

4.   Leaves coarsely and deeply serrate, auriculate at the base ..............Syn. rhabdos

–   Leaves finely and shallowly serrate, exauriculate ...............Syn. glomerata

5.   Leaves broadly ovate, ovate-lanceolate or elliptic ...............6

–   Leaves narrowly lanceolate or narrowly oblong .................9

6.   Phyllaries 5; disc florets 2 or 3 .........Syn. penantha (Synotis pentantha is widely distributed in Bhutan, China (Xizang, Yunnan), northeastern India, Myanmar, Nepal, and northern Thailand (Fig. 5). It grows in open forests, on forest margins, or among scrub at elevations between 1100–2400 m above sea level)

–   Phyllaries 7–10; disc florets 5–10 ....................7

7.   Capitula 3.5–4 × 1.5 mm; phyllaries 6–8 mm long; marginal florets ligulate, conspicuous .........S. triligulata

–   Capitula 4 × 2.7 mm; phyllaries ca. 4 mm long; marginal florets minute, filiform .........8    

8.   Stems, veins on abaxial surface of leaves and peduncles all densely glandular fulvous-pubescent ...........Syn. saluenensis

–   Stems and leaves glabrous; peduncles pubescent .............Syn. simonsii

9.   Leaves auriculate at the base ...................Syn. auriculata

–   Leaves exauriculate ................Syn. baoshanensis