You can document her school attendance, talk to her teachers about your concerns, take her to the doctor, etc. If she doesn't get her regular medical and dental exams, document that (and take her yourself). It's hard to prove that she isn't getting regular meals, but you can always note things she says in a journal. If she isn't kept clean, her teachers can tell you about that too.
Be very careful that you don't ASK her about any of your concerns. Just document what she tells you.
Also, this is really not any of your business, legally. Don't get involved, other than to make notes. This is between your husband and his ex. Judges frown on step-parents who overstep.