New dentist using existing Post

18 Months ago I had a post put into my upper first molar.  I moved and so I found a new dentist to complete the work.  Last week he prepped the tooth for a crown and made an impression for the crown to be put on next week.   This is a two part question.  1. after filing down the tooth he found a new cavity on the side of my pre-molar.  This did not show up in the original x-ray, and when he showed me a picture of the cavity it was so big and black that I now wonder if it was some sort of dye left over from the mold that was just made for my tooth.  During my next visit, he plans to put on the crown, and do fillings on the premolar, 2nd molar and a molar on the bottom. What should I ask the dentist to do, that will confirm that the tooth actually has a cavity?
2. If the new dentist is using the existing post I had put in about a year ago, would I pay full price for a crown to be put on?  Or could this be called something else to help bring down the cost?  See Profile pic for x-ray
Clearly I am a little jaded.
Thanks

[Jerome Tsang, DDS]

1)  Confirm that there is a cavity on the bottom teeth?  Look at the x-rays and look for dark spots between your teeth.  If not, then you'll have to visually see the cavity with a mirror.  Cavities can form anywhere in the mouth and x-rays are used to see certain areas that we can't see with our eyes but we need to do both a clinical exam and x-rays to put all the information we see together to come up with a proper diagnosis.

2)  Yes.  There should be no charge for a new post, however.  I'm not sure about bringing down the cost.  You may need to discuss it with your dentist but you should always be careful about trying to reduce costs.  Some dentists may not, others may.  Unfortunately dentists are people too and they come in both good and bad.  A bad dentist may lower his price but he may also cut corners or there may be a drop in quality.  The most important thing is to have trust with your dentist.  If you find one you trust and you feel has your best interest, then you would feel that the cost would justify the quality of the treatment you receive.