1. You don't dread going into work every day. Some people do, especially on Sunday nights when they've got the whole week in front of them.
2. There are clear opportunities for growth, a.k.a. promotions. Some companies don't have that. Especially smaller ones where there's not a lot of turnover as far as management positions go.
3. You have a healthy work-life balance. Meaning you mostly enjoy your job . . . you aren't constantly stressed out . . . and you have free time outside of work.
4. You like the people you work with. It's normal to have a few you don't click with. But in general, you should get along with your co-workers and not hate being around them. The most important one is your boss.
5. You're learning and adding new skills. Studies have shown it's one of the most important long-term things to think about. So if you just do the same thing every day and aren't adding any job skills, it might be time to look for something more challenging.