Legal Question in Landlord & Tenant Law in Illinois

Based on what you've given us to work with, the pertinent questions appear to be:

1. Can the landlord use the check she wrote as a contract? "Maybe." You don't say if she moved into the apartment, or whether the landlord presented her with a lease to sign, or if there are other written papers that could, taken together with the check, prove there is a rental agreement. There may be other facts of importance, like how long ago that happened and whether the landlord already cashed the check.... Without a signed lease it may just be up to what the landlord can prove, and yes leases can be unwritten (oral). Proving unwritten agreements is usually more difficult because it is often a case of "he said, she said".

2. Could you lose the deposit? "Yes." What happens to the deposit will most likely depend on whether a court decides there was any kind of rental agreement.

Money is dear today for many people, but a good lasting relationship is, as the commercial says, "priceless". You are clearly putting things into priority to save your marriage and it sounds like you and your wife are talking. Based on your zip you live in a small town and it is possible most people know each other, and "talk". This can help or hurt if your wife told other people what she was doing; make sure the two of you talk through all the possible facts. As to the landlord, I happen to believe talking things through is he better approach, in part because you're letting the landlord know not to expect to start to receive rent and therefore to find another tenant as soon as possible, but if you don't know how the landlord will react and needs the money badly, he or she might still make things difficult. Good luck but keep your sights on the most important things.