If you have a few people sorting it is time consuming and dirty work. This is done in some countries where rubbish moves along conveyors and is sorted in to its different types. It is much easier if we can sort materials at source so that later on it is already half done.
This sign
tells us if a material is recyclable. Often it has other information to tell us
what type of material it is - look here for plastic
sorting .
Look here for metals which can be sorted according to type. More info here.
How can we make it easier to get people to recycle? Look here. Or here.
Don't forget, a picture can tell you something even if you can't read.