The quick answer is that she will clearly let you know when the time is right. The contractions will be painful (or at least very uncomfortable) and they will be regularly spaced over time.

The following text explains the different types of contractions during a pregnancy.

Braxton Hicks Contractions:

The muscle of the uterus need to flex to strengthen themselves and (later on) to gradually soften the cervix in preparation for the actual birth. This spontaneous and irregular contracting of the muscle begins early in the pregnancy, but is only normally noticed by women from about the 20th week.

A British doctor named John Braxton-Hicks first described this process some time in the late nineteenth century, this is why they carry his name. These contractions should normally be painless, which differentiates them from false labor.

These contractions become more frequent and more pronounced towards the end of the pregnancy. The shapes that are formed by the belly are incredible as you can see arms and legs bulging out as the muscles contract. You will think that the baby must be very uncomfortable to be squashed in this way, but don't worry, he/she handles it just fine.

Contractions can sometimes be brought on by light exercise or dehydration, so it is important to keep drinking plenty of fluids so that the uterus does not become over-irritated.

Before 37 weeks of pregnancy, if contractions become regular or painful or are accompanied by any type of discharge you should contact your doctor immediately.

During the last few weeks of pregnancy, the Braxton Hicks contractions become more rhythmic and appear much more frequently. This period is called pre-labor during which time the cervix is gradually stretched and softened in preparation for the birth.

When these contractions begin to be painful in the lower abdomen and arrive often, the term "false labor" is used to describe the phase. You can only tell from the outside that this is not true labor when the contractions are irregular over time and eventually stop. The crucial difference is that the cervix itself does not actually change.

When the contractions are regular, rhythmic and painful or very uncomfortable, the chances are that the true labor stage is being reached. Depending on your country, the doctor will advise that you go to the maternity ward when the contractions last for about a minute and are arriving every ten or fifteen minutes.

When you get to the maternity ward, the first thing they will ask is how much time there is between contractions. They will then check the cervix internally to discover whether or not a change has occurred. If the cervix has started to open, is thinned out and has moved into line with the birth canal, then true labor has been reached and you can start to get prepared for the birth.