Have you taken her to the vet to be sure this isn't a medical issue? If this has started to happen more and more I'd suspect a UTI or other problem, so this would be the first step.
If she doesn't have a UTI or other problem, then you know you can move forward with training etc. instead. It might be as simple as treating a UTI though.
Most dogs actually need to be trained to give a signal to go out, if that is what you wish to have them do. There are several posts from the past on this, training to use bells, etc. that may be of help if you wish to train her to do this. None of my dogs give signals.
It sounds like overall, though, that right now, she has too much freedom as she is not housetrained fully (not if she is having regular accidents), and therefore, has to be managed and trained as an unhousetrained dog. The only way to deal with this, once you get an all clear for a health issue and assuming that isn't the problem, is start from scratch. There are lots of links in the training sections I've pinned to this section on housetraining; Ian Dunbar's free download book also has plenty on housetraining so any of those things should help. I'd start from square one. Put it this way: every single accident further lengthens the training process because it reinforces that this is an option for her, and every accident makes it more and more difficult to address and re-train, so managing this to halt the acidents immediately is really critical. She needs to *never* have the opportunity to make these mistakes, which means constant supervision/management, just as you would with a puppy.
Unfortunately some adult rescue dogs from some backgrounds simply never will be fully reliable for many different reasons-- and in such a case there may be nothing that can really be done but to accept and manage this (eg get rid of area rugs and never allow her in rooms on her own or with carpet unless she is on a lap, in crate, at arm's reach etc).