Thats a handy friend
Signs the mating was successful
The easiest and most effective way to ascertain if mating has been successful is observation (i.e. witnessing the event). Additionally a 'waxy plug' in the vagina can be an indicator that mating occurred, however often this cannot be seen but the mating was still successful.
Signs of pregnancy
There a number of signs that your doe is pregnant (although do be aware that most are merely predicative and do not mean that your doe is definitely pregnant).
Weight gain and abdominal swelling
Although most rats do not show obvious weight gain until the last seven days of pregnancy keeping an accurate record of your does weight before and after mating will enable you to accurately assess weight gain. You should also aim to record your does weight every few days throughout the expected pregnancy period. The shape of her body will also differ from that of other does, towards the end of the pregnancy her tummy will take on the look of a tennis ball or orange.
Please note that these signs could also indicate ill health so if it is unlikely that your doe has mated with a buck please take her to a vet immediately.
Lack of further heats
If your doe does not come into heat again when you next expect her to, this is fairly indicative that mating was successful.
Temperament change
The hormones of a pregnant doe will change drastically, and so her temperament might change. She may become skittish and she may rise to the 'alpha' status in her cage. There can also be signs of lethargy 24-48 hours prior to birth.
Hair loss around the nipples
This can occur from around the end of the second week of pregnancy.
Nest making
Although it may not occur until a couple of days before the birth (or not at all) manic nest building is a good sign that your doe is pregnant, and the birth is close. She may choose to nest in whatever suitable housing you have provided (guinea pig igloos make a good place for your doe to nest) or she may choose to make her own (in her eyes) more suitable nest. It is advisable to provide her with extra bedding so she can make a comfy nest for herself and future kittens.
http://www.fancy-rats.co.uk/information/breeding/index.php?chapter=mating