That's one way, maybe that they are receiving mail there is another, or the address of their drivers license, credit cards, etc
Dear how do I prove a tenant was subleasing?
If the subleasing is a past behavior and is not current, it makes no practical difference whether you can prove or not prove that a sublease occurred without your consent.
A tenant caught in an active sublease without the consent of the landlord is entitled to an initial written notice which factually describes the conduct forbidden by the lease and also sets a deadline date to "cure" the default.
A "cure" by the tenant will stop any more action by the landlord.
In New York City, even when the tenant does not "cure" and persists in the default conduct and forces a termination of the tenancy and an eviction proceeding, the tenant is allowed by statute to "cure" even if the tenant lost the trial.
In New York City, a landlord will not be able to evict the tenant for a breach of lease if the tenant "cures" the breach of lease after the trial ends in the landlord's favor.
"***Sec. 753. STAY WHERE TENANT HOLDS OVER IN PREMISES OCCUPIED FOR
DWELLING PURPOSES IN CITY OF NEW YORK.
4. In the event that such proceeding is based upon a claim that the tenant or lessee has breached a provision of the lease, the court shall grant a ten day stay of issuance of the
warrant, during which time the respondent may correct such breach.***"
If your tenant is currently subleasing, bring your complaint to your attorney. The attorney will prepare a Notice to Cure as required by the lease. If the tenant does not cure, your attorney wll guide you through the next steps.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.
I agree with Mr. Smollens that obtaining an actual eviction for illegal subletting can be frustrated by a late "cure" (unless the tenant of record has died or moved away with no intent to return). Moreover, New York law allows a tenant who maintains the apartment as her primary residence to have a "roommate." Even if you have not seen the tenant for a few months, some temporary absences, such as schooling, temporary work assignment or caring for a sick relative, will be excused by a court. For a rent stabilized apartment, you may find it more effective to commence a non-primary residence proceeding against the tenant of record, since that violation - if proven - is "non-curable." Doing so is a highly technical procedure and you should consult an attorney.
I am not your attorney, and I am not providing legal advice to you or any reader. Find your local bar association online and contact it for referrals of qualified lawyers in your area. I do not provide answers to legal questions not involving New York state law.
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