I assume that when you mean symmetrical you must have Markovnikov's rule at the back of your mind?
anyways,
ill try my best to explain this:
Alkenes can be symmetrical or non symmetrical
Example of a symmetrical alkene is Ethene
It is symmetrical because on each carbon that has a double bond, the constituents are the same.
The way to find out is/.... i guess.. you could draw a line down the double bond, and treat the left side and right side seperately.
If the left and right sides are equal, that is mirror images, then the alkene is symmetrical.
However,
Propene is unsymmetrical
draw a line down the double bond
One of the carbons will have 2 hydrogens attached to it,
while the other will have 1 hydrogen and one methyl group
Hence not symmetrical.
For E/Z isomerism
i really cant explain without drawing anything...
sorry