Amber is often very hard to separate from its imitations due to overlapping physical and optical properties. When no internal features are diagnostic, one can, usually, only fall back on destructive methods like the hot needle test and ether/acetone.
Copal is also a natural occurring fossilized resin which can also be artificially created. The resin has been buried in the earth for a considerably shorter period and its hardness is lower than that of amber.
The best way of discriminating between copal and amber is by placing a small drop of ether or acetone on the gemstone. This will create a sticky surface on copal, while it has no effect on amber. This is a destructive method though.
The fluorescence of copal is much whiter than that of amber.
Plastic is often used to imitate amber. Even complete insects and/or small animals like salamanders are embedded into the mould to mimic natural fauna inclusions. When the hot needle test is applied to them, it will smell acrid instead of piney.
If you are offered a piece of amber with a complete small animal at low cost, there must be something wrong as they would fetch several ten-thousand dollars when genuine.