How can I tell if a COLD cooked chicken breast is fully cooked? I poached chicken last night, (single layer of breast in a large pan, covered in water and brought to a boil, then covered the pan and then simmered for about 15 minutes. But then I turned off the burner and let it steam for an hour or so.) I can't use a thermometer anymore, since it's cold! It is white, not pink, and it tears really nicely - not tough at all. In fact, it's super tender.
There is no way of knowing. The importance of cooking for food safety comes down to log reduction of pathogens.* Without accurately knowing the exact temperature that the meat reached and for how long it was held there, you can't known if it cooked long enough to be safe.
Additionally, you can't count on color, texture, or juices to indicate how cooked a piece of meat is - acidity, age of the meat, etc can play a large role in the appearance of meat. Typically, yes, white meat with clear juices is cooked... but not always.
*: In case the source goes down some day - table of 6.5 and 7 log reduction times for salmonella, at given temperatures. Low end is 130F/54.4C 121 minute for 7 log reduction. High end is 158F/71.1C 0 seconds for 7 log reduction. That is, you will reduce the count of salmonella to 1/10 millionth at the given cook time and temperature. This is why we tell people to cook pork and chicken to 165F - it instantly kills any salmonella in the meat. Salmonella is a very heat tolerant microbe, so if we've performed a 7 log reduction on it, we've done an even larger reduction on other harmful microbes. All of this information is super useful if you ever decide to sous vide meat.
First, if in doubt, throw it out. Read this post for a full explanation.
Assuming they were properly cooled after cooking, it really depends on how thick the breasts were and if they were boneless.
If they were boneless, you cooked cooked the chicken long enough to reach pasteurization temperatures. Compare to this recipe's time recommendation:
Reduce to a simmer, cover, and cook: As soon as the water comes to a boil, reduce the heat to low, cover the pot, and let the chicken simmer. Begin checking the chicken after 8 minutes: it is done when opaque through the middle and an instant-read thermometer in the thickest part of the meat registers 165°F. Chicken will typically finish cooking in 10 to 14 minutes depending on the thickness of the meat and whether it is has a bone.
Other recipes suggest 25-30 minutes for large bone-in chicken breasts, so your answer will depend on whether the water stayed at a high enough temperature for long enough after you turned the burner off.