The babies will come when they are ready, there aren't any truly reliable signs. I once had one sow who had pelvic bones separated 2 to 3 fingers for over two weeks before she birthed, drove me crazy enough to vow never to check the pelvic bones on my sows again, it just stresses both me and the sow out too much and doesn't really tell me anything.
She is probably close (within 2 to 3 weeks) of birthing, but they could come any day. Prepare for babies ASAP, or she might catch you off guard. It would be ideal for you to check around your area if you haven't already to see which vets are exotic vets, and if there are any emergency vets who are able to treat exotics, just so you know ahead of time who to contact if she were to experience any birthing complications.
What you need to do to prepare is first make sure her cage is large enough for her and babies, a cage 10 square feet or larger. Then make sure the cage is baby proofed, you don't want the babies to be able to escape or get stuck anywhere, if you have a c&c cage this means making sure the sides are ideally 10 inches high (you can temporarily increase the side height with cardboard between the side of the cage and the grids).
Baby guinea pigs are born fully furred, eyes open, ready to run, and ready to eat solid food within minutes of being born. As such they can eat anything mum can eat. However because she has been pregnant and will be lactating, and the babies will have growing bones they need a source of extra calcium within their diets. This can be given to them simply through a good quality young guinea pig pellet (brands we recommend are Oxbow or KMS), OR adding alfalfa hay to their diet (in addition to their regular hay at a ratio of roughly 30% alfalfa to 70% regular hay), OR adding one high calcium vegetable to their daily 1 cup of vegetables (for example a couple sprigs of parsley). The third option is best for cages with mixed ages, as guinea pigs over 6 months who are not pregnant or lactating should have their calcium intake cut back, so this makes the third option ideal as it is the easiest to give to some but not others by taking them out and feeding the high calcium vegetable to the ones who need it during lap time.
Also make sure they have a good diet, a pregnant or lactating sow is at higher risk of developing vitamin C deficiency (scurvy) as their vitamin C needs increase during this time. For the mother guinea pig a good diet includes unlimited hay (as much hay as she will eat, always top up the hay before it runs out), unlimited pellets (cut back to 1/8 cup per guinea pig per day when she finishes lactating), and 1 cup of vegetables which includes a high vitamin C but low sugar vegetable such as 1/8 of a large bell pepper (baby guinea pigs don't need many vegetables so don't scale this up too much after babies are born).
Male guinea pig pups need to be separated from the females when they turn 3 weeks of age, this is because after roughly 3 1/2 to 4 weeks of age they become sexually mature. So it is important to separate them before this happens to ensure they don't impregnate their mother again. It can be hard at first to sex baby guinea pigs but it becomes easier as they age, male guinea pigs have a bone in their penis so you will be able to feel it just under the skin just above their genitals.
Baby guinea pigs can be held as soon as mum has finished cleaning them off, but don't take them out for too long too often. It is ideal to take them out and weigh them daily at roughly the same time to keep an eye on how their weights are tracking. Expect a weight loss over the first day or two, this is perfectly normal as mum won't have started producing milk yet. After that they should start having a steady weight gain.
If you have any questions feel free to ask, I'm sure there is probably something I have forgotten to cover. I hope your sow has a safe and uneventful delivery and has healthy pups.