To discern real amber from fake plastic or copal,first you can try just a few simple tests:

1.Copal (immature resin) and plastic fake amber do not hold up to solvents. Take a few drops of acetone (fingernail polish remover) or alcohol and drip it over the surface of your piece. If the surface becomes tacky, or the fluid takes on the honey golden color of the substance, you can bet it's not amber. Amber is not harmed and will not dissolve under these solvents.

2.Amber does not melt. It will burn away like incense. Copal will melt and plastic will melt leaving a horrible chemical smell upon burning. Amber smells sweet, piney and pleasant when burnt, the very reason it has been used for centuries as incense.

3.Amber is buoyant in salt water. That's why it is easy for locals on the Baltic Coast to find it washed up on beaches. Mix up about a 1 part salt to 2 parts water and let the salt dissolve completely. Drop your piece into the mixture. Plastic and copal will drop out, while amber floats.

A more complete, scientific battery of tests includes:

 
TESTS

Specific Gravity: Dissolve two tablespoons of table salt in eight ounces of water. Remove the "amber" from any mountings and drop it into the solution. If it sinks, it is not amber. If it floats, it may be amber.
Hardness: Try scratching the "amber" with your fingernail. Real amber has a hardness of approximately 2.5 on the Moh's scale. If you can scratch the specimen with your fingernail, it is not amber. If you are unable to scratch it with your fingernail, it may be amber.
Static Electricity: Place some small pieces of tissue on a flat surface. Rub the "amber" vigorously against a piece of velvet until it is warm and hold it about one-half inch above the tissue pieces. If the pieces of tissue are not attracted to the specimen, it is not amber. If tissue is attracted to the specimen, it may be amber.
Fluorescence: Place the specimen under a short-wave ultraviolet light. If the specimen does not fluoresce or if it fluoresces other than a very pale blue, it is not amber. If the specimen fluoresces a pale blue under short-wave ultraviolet light, it may be amber.
Smell: Rub the specimen briskly on a piece of cloth until it gets warm, then smell it. If the specimen emits a plastic or chemical smell, it is not amber. If it emits a mild pine or turpentine odor, it may be amber. Look out for the possibility that the specimen might be copal, which we will discuss later.
Refractive Index: Drop the specimen into a glass of mineral oil, such as Johnson's Baby Oil. The oil has a refractive index very close to that of amber -- 1.54. If the edges of the specimen are easily distinguished from the oil by either a dark outline or light halo, the specimen is not amber. If it is difficult to discern the edges of the specimen, it may be amber. Note: This test may be difficult to perform if the amber is exceptionally dark. Try to disregard the color difference between the specimen and the oil.
Taste: Wash the specimen in mild soapy water, rinse it thoroughly, then taste the specimen. If you detect anything other than the mildest taste, especially if you notice any strong, unpleasant, or chemical taste, the specimen is not amber. If the specimen has no taste (or one that is very subtle) it may be amber.
Entomology or Paleontology: If the "amber" contains an insect or other animal, try to have it identified. Most insects and animals found in real amber are extinct. If the animal is not extinct, or if it does not match those found in amber documented from the same location, the specimen is very likely not amber. If the animal is extinct and matches an animal found in amber from the same location, the specimen may be amber. Beware! There have been instances in which an insect has been manually inserted into a piece of real amber.

There are several additional tests which are (or may be) destructive to the specimen. Not everyone may let you perform these tests on a piece of "amber" that you are considering purchasing. However, if you have a specimen of questionable pedigree in your collection, you might consider the following tests:
Hot Needle: Heat a needle until the tip is red hot, then place the point into the specimen. If the needle goes in easily, or if a bad smell is emitted, or if the needle leaves a black mark on the specimen, it is not amber. If the needle enters the specimen slowly and the specimen emits a pine or turpentine smell, it may be amber.
Solubility: Place a drop of acetone, ether, or 95% ethyl alcohol on the specimen. If the area dissolves or if the surface becomes tacky, the specimen is not amber. If the surface remains intact, it may be amber.
Melting Point: If you have an oven or other heating apparatus with an accurate temperature indicator, place the specimen in the oven and heat it. Increase the temperature slowly over time to determine its melting point. If the specimen melts below 390 degrees Fahrenheit, it is not amber. If its melting point is in excess of 715 degrees Fahrenheit, it is also not amber. If the specimen melts between the temperatures of 390 and 715 degrees Fahrenheit, it may be amber.

If you encounter a specimen that looks like amber and passes some, but not all, of the tests mentioned above, you might be dealing with a specimen of copal. Copal is best described as "young" amber. Copal, like amber, is formed from the resin of trees. However, unlike amber, copal is only one-hundred to three-million years old. Copal from Kenya and Colombia is frequently marketed as "amber." There are several tests that can be performed to distinguish copal from true amber. Copal is softer than amber; it has a hardness of about 1.5 versus amber's 2.5 on the Moh's scale. Thus, copal can be scratched with a fingernail. Copal is generally brittle and sensitive to heat or sunlight. It is difficult to cut and polish, and will develop a crazed surface in a few years. Copal has a much lower melting point than amber, about 300 degrees Fahrenheit, and is soluble in acetone. When subjected to short-wave ultraviolet light, copal does not fluoresce. Additionally, the insects in copal are recent -- those in amber are generally extinct.