I Can Easily See This During The Next Fire-Nation Avatar.  And Since The Fire-Nation Is Based Off Japan

How would a person actually find out they have the ability to bloodend and if it's possible to know without training to do it.

Of the four bending arts Waterbending is most versatile and adaptable. After all, it is the Element of Change.

Waterbending is also the only element where most people can learn ALL the subtypes. (Provided they spend the time to learn.)

So to answer your first question: Most waterbenders have the ability to bloodbend but also need three things:

A deep understanding of the body and its anatomy, which is usually gained after becoming an expect healer.

Immense bending strength. Bloodbending works by dominating the water and Chi in another person’s body. A bloodbender must be strong enough to overcome their victim's Chi.

The Full Moon. Most waterbenders do not have the strength to overcome another person's Chi, therefore they need the added strength coming from the full moon in order to bloodbend.

All in all, bloodbending is a master level ability which can only be learned by very strong or master waterbenders.

As to your second question: 'Can someone know bloodbend without training?"

Answer: No. Bloodbending requires too much skill to be done by accident. (Just like you can't accidently do Calculus.) It's a skill that has to be learned and developed.

Apologies for taking so long to reply.

Bending 101

Lesson 1:  What is Bending

I love these visuals from AvatarParallels. 

Special Firebending Techniques.

[airbending] [earthbending] [waterbending]

Avatar:  Glory and Shame

Personally, I think Mike and Bryan get too much credit for what was really a team effort.  A good team balances out each other's weaknesses.  While making Avatar, Mike and Bryan were balanced out by other great writers.  However, Korra was mostly made by Mike and Bryan, so any personal biases they might have don't get edited out.  

Taking all the glory also means taking all the shame. 

I am an Earth Science Teacher who studied the four elements in university.  I hope to make a series on the science within Bending.  

Which kind of bender are you?

WaterBending101

Random Thought:  Avatar vs Percy Jackson

A while back some people arguing who was better/ stronger; 

Percy Jackson or a water bender.  

Most of them concluded that Percy Jackson was stronger, but I always thought this was an unfair comparison.  

First, demigods in the world of Percy Jackson are primarily warriors who only occasionally/ rarely use elemental powers.  Second, the rules and limits in Avatar are very clear, whereas a Demigod's power doesn't have any clear limits.   The two worlds treat magic so differently that it feels unfair to ask "who is better".  You might as well compare “Dragonball” to “Harry Potter”.