Answer: The simplest way is to determine if a strain is mutant is observing morphology, growth rate, double time, etc but it is accurate if you can prove if the strain is deficient in one aminoacid or can't metabolize lactose, etc.
Explanation: A wildtype strain functions normally, for example, can metabolize as a carbon source, glucose, lactose and other sugars, can synthesize all the aminoacids requered for protein synthesis, etc. If a strain suffers a mutation and it is inheritable, the strain become a mutant. Since several mutations can be silent ones, only those that interfere with a process, can be assesed easyly.
For example, if you have several strains and put them in a lactose medium, but some of them cannot growth means that are lactose mutants. Those strains could carry a mutation in genes that encode lactose degrading enzymes or in regulatory genes of the lac operon, etc.
