Are two samples polymorphs of the same compound

Historically, most polymorphs have been discovered by serendipity, rather than as the result of a systematic search (e.g. Tutton 1922 Cholerton et al. 1984 Chemburkar et al. 2000 Lommerse et al 2000). On the one hand, this testifles to the general unpredictability of polymorphism, but on the other hand, it also testifles to the intellectual curiosity and powers of observation and analysis of the discoverers. What generally characterizes polymorphs of a compound is that some or all of their properties will differ. Thus, in principle, an investigator armed with the knowledge that a particular compound is chemically pure, can employ almost any sufficiently sensitive physical measurement to determine if two crystalline samples of that compound may be polymorphs or not. As noted earlier, however, in practice the full characterization of the polymorphic behaviour should involve as many techniques as possible (e.g. Chiang et al. 1995). Previous sections outline many of these physical measurements and provide examples of the distinctions between polymorphs. In addition to those, however, the answer of McCrone (1965) to the question posed here is worthy of repetition, since it demonstrates the principles and the relatively simple and straightforward techniques that are involved. 

it is necessary to eliminate tautomerism or dynamic isomerism by determining that the two materials give identical melts. X-ray diffraction can determine that crystal strain (which can be mistaken for polymorphism) is not a factor. Then, McCrone 

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