Rat's gestation is 21-23 days on average. So you are on the 21 day watch since you got her.
Can you get a hold of a digital scale? One that weighs in gram increments like a postal or food scale? If so you want to weigh your new girl every day at the same time, and record her weights. I put mine in flat tupperware with fleece inside to make it less scary, and put it on the scale. You will see a very significant gain, depending on her age and size. It will increase most in the 7-10 days.
I did up a chart on 2 of my young rescue girls weight gain...Bronwen was my first and only 3-4 months old and still growing, so I needed to differentiate growth weight from baby weight.
Bronwen's Weight Gain (3-4 months old)
Aug. 11 - 267 g
Aug. 14 - 279 g (8 g gain)
Aug. 15 - 289 g (10 g gain)
Aug. 16 - 302 g (13 g gain)
Aug. 17 - 319 g (17 g gain)
Aug. 18 - 329 g (10 g gain)
Aug. 19 - 346 g (17 g gain)
Aug. 20 - 354 g (8 g gain)
Aug. 21 - 364 g (birth) (10 g gain)
Sept. 14 - 278 g
Lilith's weight gain (pregnant at 5-6 weeks old)
Aug 4 - 145 g
Aug 5 - 154 g (+9)
Aug 6 - n/a
Aug 7 - n/a
Aug 8 - 174 g (+ 20 over 3 days)
Aug 9 - 171 g (-3, there is often a stall or even dip)
Aug 10 - 189 g (+18)
Aug 11 - 212 g (+23, I thought she had hit maximum density)
Aug 12 - 217 g (+5)
Aug 13 - 231 g (+14), no she managed to keep packing it on)
Aug 14 - 241 g (+10)
Birth of 8 baby boys
total gain in 10 days was 96 grams
Other signs of pregnancy are prominent nipples from grooming off the hair around the nipple f(for pups ease of nursing), possible nippiness or biting, eating everything in sight, excessive sleeping and that typical pear shaped belly.
If you want to treat your girl (young I imagine) as pregnant it won't hurt her in the least. Add more proteins to her diet like fish, chicken, and eggs. Add lots of good healthy veggies. Make sure her staple diet is a good one (no seed mixes or crappy quality blocks). Handle her daily to get her used to you before the birth because a socialized and happy mom makes for happier, better babies right from the start.
When the birth is soon, make sure you have her in a cage where the wire part sits on the base, not rests into the base (tiny babies have been known to get stuck between the bars), and make sure the bar spacing is adequate for smaller rats. Remove any houses you were using (cardboard boxes are fine but they will smell badly in just a couple of days. I know breeders who prefer not to offer a house so that they can make sure the birth is progressing normally, and the pups are okay. I also like to just give them nesting material and no box or hide (some hides are heavy and can pin and kill a pup if mom bumps it onto them). I use carefresh or papertowels for the actual birth, tissue like facial or toilet paper can stick to wet pups and is very difficult to remove without tearing their tender skin.
Once the babies are born look for milk bellies (whitish, yellowish bands across their abdomen) showing that they are nursing properly. I also remove any soiled bedding at this time as I count the babies. I handle my rescue eepers from Day 1, every day, but just for a few minutes at first. Without any fur they cannot regulate their own temp, and will get cold away from the nest. Mom will still need to come out and run around, at this point I let her out a couple/few times a day for about 20 mins a time. This is when I handle the pups, sexing them and just getting them used to human touch. Rat mom's do not eat their offspring if you touch them, they aren't like a lot of other rodents.
This should get you started.
Can you post a pic of your girl so we can figure out how old she is and the likelihood of her being pregnant rather than just a chubby growing girl?