It's usually coming through cracks in the wall, a leaking window seal, or seeping up around the edges of the slab.

In addition to checking smoke alarms, fire extinguishers, I also check for plumbing leaks and it also allows me to get the lay of the land on the tenant. 

Depending on what type of loan you try you may be able to rent it at closing BUT HOWEVER If it's not legal and you have a fire, or tenant complains to city, or the roof leaks... no permits... you can't rebuild... the city red tags the building.

IE if roof is leaking fix it that type of thing.in practice very few redemptions are made by the owner who lost the property I only know of one personally and i suspect there are not more than a hand ful in this state.when i was buying a lot of property in Bham.. we could buy redemption right title insurance.

I had the stove and fridge break as well as the roof leak within the first year.

., etc....and even with lots of skill and preparation, things will still go wrong (vacancy, plumbing leaks, bad tenants, etc.)

Bad roof design keeps it a leaking plus other chronic issues inherent with a 107 year old soft brick building. 

The waste and overflow of her bathtub cracked and was leaking water downstairs every time she would shower!

Recently, there was a water leak in our unit that affected several other units and some common areas.

(There was a leak under the toilet causing a puddle that the tenant decided not to wipe up for months, causing a hole in the ground that even affected the foundation.)