1.שו”ת הר צבי יורה דעה סימן עט
Rabbi Tzvi Pesach Frank (1873–1960). Born in Lithuania, he lived in Jerusalem. Har Tzvi is the name of his collection of responsa on the Shulchan Aruch.
Question: May a lab that is testing chocolate to find out if it has milk and meat in it, run the test by cooking the chocolate since there is a prohibition from cooking milk and meat together?
Answer: Yes, it is permitted to run the test by cooking the chocolate.
Reason: Because the prohibition on cooking milk and meat together is only when there is a fear/risk that one will eat the food, but here the whole purpose of the test is to find out if it can be eaten or not. It is clear that the chocolate will not be eaten if it turns out to have both milk and meat in it. Also, Frank follows the opinion that once something is cooked, reheating it does not cook it. The halachic term for this is אין בישול אחר בישול ein bishul achar bishul (there is no cooking after cooking) so you are not actually cooking meat and milk together.
2.שו”ת יביע אומר חלק ז – יורה דעה סימן ה
Question: May a lab that is testing chocolate to find out if it has milk and meat in it run the test by cooking the chocolate since you are prohibited from cooking milk and meat together?
Answer: Although there are reasons to prohibit running these tests, you should be lenient and allow it.
Reason: He brings the same reasons as the previous response to this question and quotes him in his answer.