How to handle tenant damage from previous ownership?

[Landon Clipp]

I recently acquired a property that has a group of tenants who have been living there for about 3 years. I went to the property today to do various things and one of the tenants told me that about a year ago, their upstairs toilet clogged and flooded. The water drained through the floor, through the kitchen cabinet and onto the kitchen floor. It caused damage to the wood above the cabinets (to where you can see through the ceiling) and possibly further damage that can't be seen. Due to its hidden location, this issue was hard to see without someone pointing it out and did not come up in the inspection. 

Since I have no evidence of when this situation actually happened besides the tenant telling me when it happened, how do I approach deducting the repair costs from their security deposit when they move out? The damage is obviously from tenant negligence, but if they fight it in court it would be challenging to prove they were the ones who did it. Perhaps I should deduct and see what happens?