It is natural for couples to want to keep tabs on each other to some degree, but there is a difference between loving attention and jealous suspicion. If one partner in the relationship always needs to know where the other person is, what she is doing and whom she is seeing, it is a sign of unhealthy obsession. A partner who questions his significant other with suspicion, demands unreasonable levels of detail in her answers and does not believe what she tells him is showing signs of jealousy.
A person who checks her partner's personal correspondence to see who is emailing, texting or calling him is displaying jealous behavior. There is no reason for someone who trusts her partner to check up on him in this fashion, especially without his permission. This kind of spying can escalate to secretly following him around and even seeking a private investigator to try to catch him in an act of infidelity. It is normal for people in relationships to maintain some element of privacy, and invading that privacy through spying is a sign of jealousy.
In a healthy relationship, both partners keep their other friends and continue to make new friends. If someone does not want his partner to spend time with other people or, in extreme cases, even talk about other people, it is a sign of serious jealousy. A person who is insecure in his relationship and fears that his partner will leave him for someone else might want to stop her from interacting with others. She cannot decide she prefers one of her other friends if she has no other friends.
Jealous people seek constant reassurance that they are loved, their significant others are faithful and nothing is wrong between them. They may become angry or upset if their partners do not return their phone calls immediately, taking it as a sign that their partners are lying to them or do not want to talk to them. They may withdraw or lash out if someone else seems to find their partners attractive, worrying that this other person will try to break up their relationships. People who see these everyday occurrences as undermining their relationships are exhibiting signs of jealousy.