Not all diffused thinners have miniturization in the donor zone. Some do and and some do not. Those that have it, do not make good candidates for HT surgery IMHO.
Without a doubt you need your entire scalp evaluated under a digital scope. The scope will magnify your hairshafts for comparison purposes. The doctor then compares hair samples looking for loss of hair shaft diameter when compared to hair shafts of "terminal" hair. Miniturization is in fact loss to hair shaft diameter. It cannot be readily confirmed from pics, especially in the early stages. It has to be evaluated under a scope unless the loss of caliber is so advanced that it can be readily seen by the naked eye.
If loss of caliber is evident, the implication then is that DHT is affecting those hairs in that particular region of scalp. Transplanting hair that is DHT receptive will not last and can be lost in the future, just as much as other hair affected by DHT. It is not considered terminal hair.
Another indication would be older men in your family history that show evidence of donor zone thinning.
I use the scope myself and it can be very helpful for younger men to know if Propecia is working by again comparing hair samples over periods of time, usually every six months.