Hey Mark,
CAP RATE= NOI/PURCHASE PRICE OR NOI/CAP RATE= PURCHASE PRICE
NOI= GROSS INCOME - EXPENSES
So, you should look for the NOI of the deal you're currently analyzing and divide by the purchase price to get your cap rate OR if the listing agent provided "their analyzed cap rate" (which is incorrect half of the time" i would do NOI/CAP RATE SHOULD EQUAL PURCHASE PRICE.
How do you know you're buying at a "correct" cap rate?
1. Look at sold comps in the exact same condition within the last 6 months
2. Contact commercial brokers and ask them what the zip code cap rate is
3. Contact commercial property appraisers and ask them what the area cap rate is
These will most likely all vary and you will have to come up with an average.
From experience big cities are going around 4-6%.
Now, cap rate is really just a way to determine the value of the building, at the end of the day what's really important is that you're cash flow positive after expenses and debt service.
Hope this helps and good luck!