In "Northern English", the phoneme /ʌ/ of RP and of general "Southern English" does not exist; it is instead part of the phoneme /ʊ/. Historically of course, this is part of a complex split of post-Great Vowel Shift English /uː/ and /u/, usually dubbed the FOOT–STRUT split, named after the lexical sets for /ʊ/ and /ʌ/ respectively. This Wikipedia article on the phonological history of English high back vowels shows how the FOOT–STRUT split emerged and gives a brief guide to which orthographies represent which phoneme: in short, the spellings "o" and "u" usually indicate /ʌ/.
As this is a clearly defined and well-documented development of the English language, it is found in dictionaries and other authorities which are consulted by learners of the language. These phonemic renditions of English words are of course invaluable for second-language learners of standard English (whether that is Received Pronunciation or General American, which both contain the split).
However, the necessary skills in listening out for these phonemes requires the development of phoneme awareness. From copious studies on children, phonemes further removed from the "native" phonemic system are more easily distinguished (e.g. foreign accents are more readily distinguished by primary school children than native accents according to Floccia & Goslin ), but that sensitivity to accents decreases with age (see this 2012 article from Cristia & Floccia).
With novel phoneme distinctions for second language learners then, three situations are generally identified (as per Eckman & Iverson (2003)):
- Both phonemes are novel.
- One phoneme is novel, but the other phoneme is present in the first language.
- Both second language phonemes are present as allophones of the same phoneme as the first language (hence the phonemic distinction is novel whilst the phonetics are more-or-less familiar).
For native Northern English speakers trying to negotiate the FOOT–STRUT split, the second situation is applicable. The problem of language transfer, especially interference from the orthography in this case, is especially relevant. Both are associated via the "interfering" similarity with the first 'language' and through the orthography with the one phoneme /ʊ/.
Interestingly, as with the comment, when /ʌ/ is compared with phonetically similar vowels from different language systems where the possibility of interference is so low as to be negligible, e.g. Russian's [ɐ]/[ʌ] or Korean's [ʌ]/[ɘ], the native language transfer mapped it to a different vowel completely, Northern English's /a/. This shows the difference between phonemic and phonetic similarity very clearly: it is very likely that your familiarity with written English and the close affinity of Northern and Southern English that you associate the sound of [ʌ] with /ʊ/ at all; otherwise you associate it with /a/.
Incidentally, while the two sounds [a] and [ʌ~ɐ] are allophones in Russian, in Korean the two are contrastive phonemes (with great ramifications for the grammar too), although of course the phonetics make [ʌ] lean toward [ɘ] or even [ɔ] - there is also a moribund length distinction creating two allophones in quality. In urban Southern English, data seem to suggest change in both STRUT and FOOT, although it seems to be a bit of a chain-shift: STRUT is moving towards [ʌ] (whereas it had been closer to [ɐ]) being backed and raised; while FOOT is being fronted towards [ɵ], as Torgerson, Kerswill & Fox (2006) report.
In short then, perhaps combining your knowledge of English orthography (i.e. its spelling with a single "o" or "u" and occasionally with "oo") with a recognition that it sounds a bit like /a/ will help you to isolate the STRUT vowel.