You're standing in the dairy section of your supermarket. You read 'Kosher Gelatin' on the Ingredients List of a container of yogurt. You've heard that 'Kosher Gelatin' can come from pig skins. But you've also heard that it comes from fish bones. How can you be sure that the gelatin in the yoghurt you are holding is halal?
The Answer is more complex than you might otherwise think.
Nationwide (U.S.), there is no legal definition of nor regulation of 'kosher'. The Constitutional Principal of Separation of Church and State prevents government from passing laws concerning the regulation or establishment of religion. This means that, legally, any food company can call and label a food product or any ingredient used in that product 'kosher,' simply if the manufacturer believes that it meets that standard for 'kosher'.
There is gelatin available nowadays from kosher-slaughtered animals, but it is usually identified by something more than the mere phrase “kosher gelatin.” Fish gelatin is usually identified as “fish gelatin.”
The OU (Orthodox Union) estimates that around 90% of the gelatin on the market is porcine - made from pigs - whether pig alone, or pork gelatin mixed with gelatin from other animals such as cow or fish.
A few lenient Rabbinical organizations (poskim), and independent rabbis allow gelatin coming from the non-edible parts of a non-kosher animal, such as the pig, or kosher non-slaughtered animals to be called kosher. If part of an animal is edible, such as meat or skin, then it is widely accepted that this part cannot be used to make kosher products. This means that in theory, pig bones can be used to make kosher gelatin. In practice, however, the "higher-quality gelatin made from bones is reserved for the photographic industry."
"Animal bones and hides are considered inedible and “kosher” even if they come from a non-kosher or non-slaughtered animal. ... One exception is that the hides of domesticated pigs have the halachic status of meat, are considered edible and are most-definitely not kosher. Thus, even those who argued that gelatin made from the hides of beef or from bones is kosher, would have a harder time defending that position as relates to gelatin made from pig hides." The Orthodox Union (OU) and other Orthodox affiliated certification agencies reject this arguement, and do not permit kosher gelatin to come from pig (or non-slaughtered cow) bones.
"... A similar argument has been presented, and rejected, for permitting gelatin produced from non-kosher hides and bones. Since, at the end of the process, the gelatin is no longer pagum (inedible), we cannot accept its mid-process pegimah (inedibleness) as sufficient grounds for permitting it." Kosher Gelatin in Yoghurt
For a long time, yogurt certified by the mainstream and Orthodox Kosher certification agencies in the U.S. was made without gelatin. However, some lenient certification agencies allowed gelatin from non-slaughtered beef bones into certified yogurt products. This situation bothered some kosher authorities, and after many years of behind-the-scenes work they were finally able to convince the lenient kosher certification agencies to stop this practice.
When this stopped, the yogurt companies did not want to change their recipes, so they began working with the kosher agencies to produce kosher animal gelatin. Kosher gelatin for yoghurt (and marshmallows) that is certified by the OU, comes from the animal hides (skins) of kosher slaughtered animals in South America and is manufactured by Norland Industries and Glatech Productions. Gelatin produced from kosher slaughtered animals is not considered 'meat' and may be eaten with milk-based yogurt.
THE BOTTOM LINE
Look for a kosher or halal symbol
Most truly kosher gelatin is usually identified by something more than the mere phrase “kosher gelatin.” It is important to look for a halal or kosher symbol on the product packaging. If the Ingredients List just says 'kosher gelatin' but doesn't provide a kosher symbol on the front of the package, you can be sure it contains pig gelatin.
"K" means nothing
NEVER assume that the letter "K" on a food package means that a product is kosher. The letter K is not copywritable under U.S. Law. This means manufacturers can use it whenever they want, whether or not a product contains pig gelatin.
You can also look for the term 'fish gelatin' on the food's Ingredients List. Under U.S. Law, manufacturers cannot mis-state the animal source of products listed in the Ingredients List, therefore if an ingredient is listed as 'fish gelatin' it must come from fish, not from pig or cow.
If a product lists 'Kosher Gelatin' on its Ingredients List, and is also certified by one of the Kosher or Halal certification agencies listed above, then that gelatin likely comes from a kosher/halal-slaughtered cow or from fish.
Kosher certification agencies, and their symbols
Halal certification agencies, and their symbols
Kosher v. Halal: What are the differences and similarities of kosher and halal?
Kosher: Under-serving Muslim Consumers
A comparison of kosher and halal animals
 First Ammendment to the United States Constitution.
 Kaplan, Rabbi Benyomin. "What is “kosher gelatin”?" Ask Moses.com. http://www.askmoses.com/article/555,2095418/What-is-kosher-gelatin.html
 Cohen, Rabbi Dovid. "Gelatin Revisited," OUKosher.com. http://www.oukosher.org/index.php/common/article/gelatin_revisited/
 Image: Gelatin Production. http://en.wikipedia.org/wiki/Image:Materials_Used_in_Gelatin_Production.svg
 Cohen, Rabbi Dovid. ibid.
 Rambam, Hil. Ma’acholos Asuros. 4:18
 Cohen, Rabbi Dovid. ibid.
 Kuber, Rabbi Mordechai. "What Could Be Wrong With…….L-Cysteine?" OUKosher.com. http://www.oukosher.org/index.php/common/article/what_could_be_wrong_withl_cysteine_by_rabbi_mordechai_kuber/
 Gordimer, Rabbi Andrew. "Culture for the Masses: The Complexities of Yogurt Certification," OUKosher.com. http://www.oukosher.org/index.php/common/article/culture_for_the_masses_the_complexities_of_yogurt_certification/