It's never nice when an employee leaves, especially if they are usually very hardworking and have delivered a lot for a company. Some employees are a great loss, and recruitment can be very time consuming and costly. It's within all employers' interests to look after their employees, help them develop and maintain long-term opportunities within the business.
1) Work starts to slow or suffer and they aren't their usual selves.
It's a particular shame but also quite a big and obvious sign that something is wrong, when an employee who normally works very long hours, delivers high quality work and always hits deadlines, starts to slow down. They may start to continuously leave on time (although this isn't always a bad thing, they are entitled too), they may start to care less about their work even if they care a great deal. You have to remember it may affect them rather personally and emotionally if they start to 'slack' with their work or care less about it. Most employees want to achieve and do well in a business; most want to do a good job. When they reach that point of exhaustion and are considering leaving they may also feel they are letting themselves down.
2) They stop volunteering. This could be to build a desk for the new office, work late on a project or another situation where they normally don't mind going the extra mile.
3) Excessive complaining: they are reaching breaking point; everything becomes a problem and a frustration. In most meetings and most discussions they aren't happy about something, it's tiring them and they are heading towards the door.
4) Longer breaks. Are lunch breaks getting longer, are they turning up later than usual for work? The signs are there.
5) Clients or customers are less satisfied. Although there are many reasons for client or customer dissatisfaction, it could be that an element of a situation where a client is normally very happy, is that your employee hasn't made them as happy as usual. Possibly because they have been attending other job interviews and are preparing their resignation.
It must be reminded that even if employees are leaving, most will not let their work suffer. Some of the above situations above may not be as obvious as you think.