How do you know if two language varieties are dialects of the same language or dialects of different languages altogether?

Answer 1

The linguistic standard is called lexical similarity. Generally speaking, if 2 languages share a lexical similarity of more than 85% (such as Norwegian and Danish), then they are considered dialects of the same language.

Answer 2

Two language varieties are typically considered dialects of the same language if speakers can generally understand each other with minimal difficulty. If mutual intelligibility is lacking and communication between speakers is difficult, the language varieties may be considered dialects of different languages altogether. Factors such as shared history, cultural identity, and political considerations can also influence classification.