How can I figure out if a water spot on my ceiling is due to an active leak?

I recently bought a house with a rather large water spot, about four feet across, on the ceiling. How can I tell if it is due to a current, actively, or an older leak that has since been addressed, but that the previous owner never cleaned up the water spot? I'm having some shingle replace today, but I'm wondering if the problem has already been taken care of or if I need to have it more thoroughly investigated.

Submitted February 20, 2015 at 08:36PM by bruce656 http://ift.tt/1zTdf3P