Standard Mandarin's monophthongs in the usual five vowel analysis /i, u, y, ə, a/ (Pinyin i, u, ü, e, a) [and even if you include the apical vowel /ɨ
/ or /ɹ̩~ɻ̩/ or /z̩~ʐ̩/, Pinyin i] mean that /o/ and /e/ are only present in diphthongs /ou̯, jou̯, wo, je, ɥe, ei̯, wei̯/ (Pinyin: ou, iou, uo, ie, üe [xue, jue, que, yue], ei, uei [written as wei or ui]), bar ...
RP's DRESS vowel is [ɛ]. Its FACE diphthong could be analysed as [ei] or [eɪ] (depending on whether the syllable is open or closed and whether the speaker is a HAPPY-tenser), in which case [e] occurs in diphthongs but not alone. Mind you, those diphthongs could instead be regarded as starting at [ɛ].
You can listen to some comparisons on Forvo here:
세로 vs 새로
I'm not an expert, but I find it's possible on a blind listen to consistently identify which is which! My mental model:
ㅐ is more open. It sounds largely like "eh" in English.
ㅔ is more closed. Like "eh" mixed with "ee".
Although many Koreans may ...