A simple “Burn Test” is used to differentiate between PLA and ABS.
When a PLA filament is exposed to fire it catches fire and you would notice the following
(a) Blue flame
(b) A dull flame
(c) The filament melts like wax and drips down
(d) On extinguishing, it would retain its orignal colour.
Whereas when a ABS filament is exposed to fire, it too catches fire and you would notice the following
(a) Orange Flame
(b) The flame is very bright
(c) the plastic melts but the way the PLA does.
(d) On extinguishing the flame you would see that the end is charred black.