Bad flatmates take and don't replace until you call them out. They make and break commitments, or wait to be called out before doing their part. You'll need to prompt them to take action because a bad flatmate communicates late or infrequently about plans affecting others, and will probably only react after general issues come up.
Sometimes, they could leave you worried about making rent and will probably have to be reminded of debts, but mostly catch up eventually, after stressing you out.
Bad flatmates are typically fairly independent both in times of lack or plenty, and have been known to move out without giving much notice for others to make plans.
A good flatmate uses and replaces things before you notice, but still tells you anyways just so you don't get surprised. They make and stick to commitments, or renegotiate them early when things come up.
Proactive and open about general concerns, a good flatmate communicates early and often about plans e.g. rent, chores and quickly resolves general issues to maintain harmony. They are open about thoughts and feelings, interactions with flatmates and propose or are open to early conflict resolution.
Speaking of rent - good flatmates are rarely or never late on rent without advance notice. They keep records and pay back everything they owe - being independent in times of lack, and generous in times of plenty.
Finally, when the chapter is over, good flatmates discuss move out plans ahead of time with up to a month's notice, and can commit and stick to a move out schedule.
Now, meet the "Flatmate from Hell"