The obvious first indicator it is time to fire someone is a lack of productivity. Sometimes the productivity gradually drops off and sometimes it is all of a sudden. As long as you double-check that you’re communicating your expectations clearly and that those expectations are realistic (typically the case when all other members of a team are able to meet them), the odds are the problem is with the employee and it is time to let them go. This goes double for employees who always seem to have excuses for their lack of productivity: waiting on others to get their work done, unaware of what is going on in general, never feeling “100%”, making the same mistakes repeatedly, etc.
The other big indicator is also fairly obvious but can be tricky: absenteeism. This employee is frequently missing in action, whether it be a situation where they are in the office but never at their desk or a situation where they are out of the office for the “next big emergency.” This indicator is quick to irritate coworkers because they can typically easily see that the employee is not getting their work done (which is confirmed when the absent employee’s work falls in the coworkers’ lap). This indicator can be tricky when it comes to employees who have chronic/extensive health issues or who have a genuine big emergency that can last for a while (death in the family, home burned down, etc). The worst situation, where you know the axe has to swing quickly, is for employees who become absent often with zero communication or excuses.