How can I tell when chicken thighs are sufficiently cooked to eat?
Pink has to do with blood, which typically runs clear once it reaches a certain temperature.  This is not a great rule of thumb for making delicious food.

It's particularly problematic with thighs, because they contain a vein that can remain red or pink well through the cooking process.  This is less so if the chicken is washed or the veins cleaned.  

The key is temperature, not color.  If you have a meat thermometer, aim for 165-170°F (74-77°C), keeping in mind that meat will continue cooking for 5-10°F (3-6°C) once it's pulled and allowed to rest for 5 minutes.  If you don't have a thermometer, aim for meat that is responsively tender but not too firm, opaque rather than transparent, and not slimy or gelatinous in texture.  Thighs are a little more tolerant than breasts when it comes to drying out because they have more fat, but nothing is sadder than overcooked meat (except maybe salmonella poisoning).