As a rule of thumb, two clear indicators are the bounce rates and spam rates related to your campaigns. If either or both of them are higher than 3% and 0.05% respectively, there is a good chance the IP will fail. Keeping them low as much as possible is how one can improve IP reputation. Apart from issues in infrastructure, if any, this can be caused due to bad list hygiene, wrong targeting of campaigns, non-engaging content/subject lines etc. Having these under control will lead to improvements in IP reputation.