How can I tell if my guitar is left handed or right?

If the guitar is right handed, the neck will go to your left with the fatter strings on top.

If it's left handed and can throw a curve ball, it will be pitching for the Yankees.

throw a ball at it and see which one it catches it with

Well it depends on the feel
A left handed guitar will be facing right instead of left.
Your right hand would be strumming instead of playing on the fretboard
The strings would be reversed

The same way that you tell if a trumpet is right-handed or left-handed .

First of all if you have to ask that question, you don't need a guitar. But to answer the question, it's all in the placement of the strings. Just as left and right are opposite, stringing is opposite.

You can tell by looking at the strings. The largest string should be at the top of your guitar when you hold in normally on your knee, with the neck in your left hand. If this is not the case, and the strings go in opposite order, then you have a left-handed guitar. Simple.

if you hold the neck with your left hand and the fatter strings are on top then its a right handed one th oposite goes for left handed ones

A guitar can be either left or right handed... the set-up is slightly different.
A right handed instrument is normally configured with the bass string on the left side of the hole when viewed from the face. A left handed guitar will normally have it on the right side as viewed from the face. The pick guard can also be changed easily to be nearest to the higher strings.
I use the term 'normally' because I have seen friends play a right handed guitar 'upside down' with the bass string on the bottom.
So, you should be able to know the configuration from the above description but remember, when you change the string arrangement, you should also change the nut (near the tuning pegs) and the bridge (near the back of the body) so the strings settle properly and don't rattle.

see what side the pickguard is on,if no pick guard,see which string is thickest