Many companies and/or designers publish radiation patterns for their antennas.
For example, here is a 3-element yagi from DK7ZB:http://www.mydarc.de/dk7zb/3-Ele-Kurzwelle/10m-28Ohm.htm
If you scroll down you'll see a plot that looks somewhat like a yellow scollop shell
outline on a brown background: that shows that radiation is maximum across a fairly
wide forward lobe (about +/- 30 degrees from the way the antenna is pointed), with
relatively little radiation off the sides and back (the lower part of the diagram.)
That's a fairly typical radiation pattern for a 3-element yagi. You can often get
higher gain at the expense of more radiation to the rear. Adding elements will
make the main lobe narrower, but you'll still hear a lot of signals from +/- 60 degrees
from the main beam, as well as some off the rear if they are strong enough.