Historically, definite articles are often related to demonstratives.
How might one characterize whether a word in a language is a definite article or a demonstrative?
This can be difficult to pin down. Demonstratives are typically used to pick something out by their location in space (or time) relative to the position of the speaker (deixis). Definite articles function as a referent-tracking device, and pick something out as having already been mentioned, or already known about. To diagnose forms as demonstratives and/or definite articles we have to study recorded utterances, along with their context, and work out how the forms in question are being used. Some languages have distinct demonstratives and definite articles, some have one set of forms that covers both functions.