How do I know if battery 100% charged?

Battery is being charged:

Battery is fully charged (charger still connected):

Battery is in use, but not being charged:

How to check the battery's status

Your iPod displays a battery icon in the upper-right corner of the screen to show approximately how much charge is left. If you have an iPod classic or iPod touch that is asleep while being charged, a larger version of the same icon appears in the center of the screen. Use the iPod shuffle status light to check the charge.

http://support.apple.com/kb/HT1384

when you plug it into a charger or iTunes, a lightening icon should show to tell you it's charging, and the plug icon meaning fully charged. if you can't plug it in I dot think there's a definite way of knowing but if the green bar looks pretty high up then it is either full or close to it.

Or if theres a plug in on the battery in the status Bar

There are lots of free apps that you can use to find the state of the battery.

For those who also come in the future, You might see the bar completely green but its not completely charged, And there is a way of knowing if its 100% charged i guess since I'm in the newest iOS the issue has been solved but how is done is while your iPod touch 5 is connected and charging and its locked hit the home button below where it says the time it will either say battery charging even though the bar is full, or it will then after that say 100% charged once it really has been.

I noticed it's a little different on my iPod Touch 6. When plugged into a wall charger it will go from empty to displaying 100% when you touch the home button in about an hour. Seemed to fast. I plugged it into my MBP and in iTunes it still says 99% and won't reach 100% for another hour or so.

Now i don't know about that, But from 99 to 100 it takes and hour or more to prevent overcharging

You can ask Siri what your percentage is