If a Dutch speaker is talking in English, telling the difference between someone from Flanders and someone from the Netherlands is generally quite easy. When the accent is strong enough, it's also possible to distinguish different regions within Flanders. For instance, techno-DJ Charlotte de Witte from Gent (East-Flanders) sounds very different from her colleague Amelie Lens from Vilvoorde (Flemish-Brabant).
Distinguishing different English accents from the Netherlands is usually harder for me, but I was recently watching this video and I could clearly tell that the interviewer is from the south-east of the country.