Shop subletted by Tanant without consent of Land Lord.

Dear Experts, Our commercial property (Shop, located in Asansol Municipal Corporation, WB) is Subletted by our tenant to a third party without my father’s (Land lord) written consent despite the fact that we have an agreement on stamp paper that the tenant can't sublet the shop. We see a third party has occupied the shop & started a business which is going for about 1 year. We heard from some source that if the third party is asked about their occupancy we will be told that they are having "partnership business with the tenant". But we are very sure that this is either a subletting or an ostensible partnership. Please help me letting know how we can get confirm that this is indeed a subletting before we sue the tenant for this. Do we need to ask the tenant/third party directly or is there any other scope to know this? We are actually finding any other way to know this. Really looking for the help as early as possible. Thank You! You can reply me the answer in my email id: