It all depends on the dog. Some are perfect loving adults already trained just looking for someone to love and of course to be loved. Some have issues which can range from not house trained, heartworm positive, other health problems, behavioural problems. Not socialized, not leash trained, not good with other dogs, not good with kids, afraid of men, afraid of women.
It all depends on what you get but if it has been in a foster home then usually the foster home can share everything about the dog with you. What I have rescued is simpley a dog who needs a loving home because the owners have divorced and neither one wants the other one to have the dog. So either I take her or she is destroyed. SHe has no issues, no behavioural problems, nothing.