If a female Muslim is not sure whether or not she is a virgin (for example because she was sexually abused as a child) is she under any obligation to mention this to her fiance before they get married?
It is commonly recommended not to tell that, it has no benefit and can merely cause in future problems. Even if --نعوذ بالله-- a man or woman has committed Zina (I mean intentionally, not as you said by being abused) then if he or she repented before Allah and decided to marry it is recommended not to tell what he has done before. However, exceptions may arise if the person is scared of his/her partner will one day understand that (e.g. there is someone who hates him/her and will try to ruin his/her life by letting the others know his/her secrets) and prefer to let her/him know before the partner will be told.
As a general rule, the bad memories and secrets are better to remain as personal memories and secrets. Propagating such things can have many undesirable side-effects. A good family that gradually move in the path of Allah may fall into troubles after the couples telling such secrets to each other, always suspecting their relations, thinking of other probable secrets not yet declared, and etc. The exception is when there is felt a necessity for confessing all or parts of such secrets.
Maybe you will feel better to consult a local religious adviser.
NOTE. All the above stated points are based on the assumption that the wrongdoer person has repented, otherwise trying to trick a person (تدلیس) in marriage will make the marriage problematic. That is the couple may depart from each other even without requiring divorce.
Lying is a grave and heavier sin than zina which is the same as ruining your earthly and heavenly life.
Note that most answers say conceal your sins (yes), but you are not the one concealing anything, but our Creator does IF you repent for your sins.
You are to keep quiet about your sins, but being abused is not committing a sin, YOU are not at fault in this case!
A person who wants to marry you, future husband/wife has all rights to know about your past even your sins. You have to answer honestly in order to THEN not destroy your MARRIAGE later on. Because everything which will be known later is mostly difficult to handle and may lead to divorce or even worse. But don't go into detail.
It is the right of a man to marry a virgin woman and vice versa, so if you are not a virgin but you lie to him, you would be deceiving your spouse.
It is an established fact that, the child is the victim in child sexual abuses, no matter what he/she thinks.
Whether you want to tell it to you fiancé is upto you. Guidance for our Prophet Muhammad (PBUH) is that if it puts your heart at ease, you should do it. It depends on how much is your understanding with your fiancé. I would personally make that abuse a part of your past and have a good relation with your to-be husband and then, later, when you think that you are comfortable in disclosing this, you MAY do that.