When they know how to prioritize important work and keeping their commitments seriously. Responsibility also means being accountable for whatever is entrusted to you. If you want to change an individual to be more responsible, consider reminding them everyday.
Trying to convince someone to be motivated about something isn't easy. You may at times have problems with this if you're on your lazy days. You just need to find more motivations to help an individual sustain their interest in keeping their responsibility
A lot of things are responsible for how responsible a person is...
The first thing is the background of the fellow person, how the person is brought up, the kind of family the person came from.
The second thing is the environment, the kind of neighborhood the person grew up and the people the person move and role with.
Also, the peer group, the kind of friends the person is keeping.
Finally, the fellow him/herself. Responsibility is a personal decision.