It started with a curious discovery. Some time ago I noticed Qingyuan cantonese (QC) using a non-Guangzhou cantonese sound "kyu" for the word district (usually keui in Guangzhou cantonese, GC) and marked it down. Then, slowly, I noticed more examples: hyu for empty (heui in GC) and gyu for geui (raise) and others. It presented a difficulty at first because the terminal -eui usually was pronounced -ui for most words (heui-hui, go; seui-sui, water; etc.); since my base knowledge of Cantonese was GC, if I wanted to say an -eui ending word I couldn't be sure if it were pronounced -ui or -yu.
I decided to do a background check on the sounds with an online dictionary and noticed that in most cases the words which become -yu in QC were pronounced -i in Hakka Chinese. To be clear, I don't know any Hakka, but it had been my theory that QC might have been influenced and inflected by Hakka (it is in a Hakka region). After checking all the examples, it was clear Hakka only helped distinguish them 75% of the time.
I decided on another tack, one that may sound a little odd: I decided to use Japanese kanji pronunciation to distinguish them. One of the unexpected benefits of having studied Japanese is that I understood the rhyme schemes of Chinese poetry better than the students who only knew Mandarin. Japanese preserves a lot of the sound and characteristics of Chinese from a particular time in Chinese dialectual development (usually said to be the Tang or Song dynasties). For poetry, Japanese kanji pronunciations show ending sounds well that Mandarin has lost, but Cantonese has preserved.
So I cross-checked it through Japanese and found that Japanese predicts the sounds that are different -eui/yu to a success rate 95% of the time. (The three sounds hyu/kyu/gyu are pronounced kyo, ku and ku in Japanese onyomi.) There was only one failed test: heui (go) is pronounced hui in QC* yet kyo in Japanese on yomi. This is just one aspect but quite an eye-opening one to test out on other GC/QC distinctions.
I found this article during my search: "In the 1990s, Akitani Hiroyuki, a linguistics professor at Ehime University (Japan), come to Qingyuan and found medieval and even ancient sound were well-preserved in Qingyuan dialect. After years of in-depth researches, Wu Qiushi holds that Qingyuan is the only one place where Mandarin Chinese of Tang and Song Dynasties are so well preserved."
I don't know what the good professor saw, but I've found one too!
*Apparently there are people/dialects who say this word "hyu" but I've never met them.