The home pregnancy test (HPT) is designed to work the same a RL version. It measures the hCG levels (which increase over time, and with the number of fertilized eggs) and will return a positive result only if those levels exceed the sensitivity of the test.
Each pregnancy is different, both in the quality of the embryo and the number of embryos, so testing may not result in a positive result until several days post-fertilization.
Standard HPTs come three-to-a-package. To unpack one, rez the box and click on it; you'll receive an inventory offer containing the test. You can also unpack the box as you would any other boxed item (the "Giver" script can be discarded).
To use the test, first rez it. You may wish to do this with your partner nearby for dramatic effect, as the results are revealed slowly.
To begin the test, just click it, and you'll receive the following dialog:
When you click Begin, a viewer will appear above the test, and it will indicate who is being tested:
Over the course of about a minute, the test result will slowly appear in the viewer. Two hearts represent a negative result, whereas three hearts (one smaller one in the center) indicates a positive result. At the conclusion, the text over the test will also change to indicate the time that the test was taken, by whom, and its result:
When the test completes, you make take it back into your inventory as a souvenir.