How can you tell whether you have a leaking filling?

I am new to the Dallas area, currently without a permanent dentist. I am in excellent oral health, or so I thought. However, I have been experiencing mild sensitivity on one of my lower-left molars, both while chewing or drinking a cold beverage. I found a gentle dentist via a web search, so I came in for a consultation. A quick x-ray was taken of the general area of the tooth in question, and she took a quick visual peek. Diagnosis: there are two molars in question (as she showed me on the x-ray) one was leaking, the other was cracked, and I need crowns for both. She showed me where the cracked molar was located, as the area appeared darker, but she appeared to be guessing on the diagnosis of the leaking filling. Both molars have fillings that are 10+ years old. How can you tell whether you have a leaking filling (since you can't tell from an x-ray)?