What is the best way to assess bud damage from cold temperatures? Drs. Tara Baugher and Jim Schupp at Penn. State University have put together a nice page with some excellent photos explaining how to assess fruit bud survival:
http://extension.psu.edu/plants/tree-fruit/news/2016/assessing-fruit-bud-survival-and-crop-potential
I would add that unless you find that more than 90% of your buds have been killed, then you should still plan on thinning. With warmer temperatures in the forecast, we expect rapid crop development and any damage should become apparent. Remember to look for damage to the pistil. This is the female part of the flower and is made up of the stigma, the style and the ovary. The normal course of events is that pollen is deposited on the top of the pistil (stigma) and the pollen grows down the style and then fertilizes the ovule to produce a seed. Generally speaking, without seed development we don’t get fruit set. Pollen will not grow down pistils that have been killed by damaging temperatures, so look for brown, dead pistils in the center of the flowers.