In the linguistic literature, phraseological units have been described as a subgroup of lexical units characterised by a set of morphosyntactic and semantic properties such as idiomaticity, expressiveness, motivation and fixedness. These properties are generally seen as gradual, i. e. their validity for individual phraseological units is a matter of degree, none of them being obligatory. With the increasing importance of corpus linguistics in recent years, there has been a growing interest in grounding all linguistic generalizations on a broad empirical basis. The present paper therefore proposes a method for systematically inferring statements on the degree of idiomaticity, motivation and expressiveness of phraseological units from corpus data. The criteria taken as relevant for this aim are derived from the realisation of the phraseological unit itself as well as from properties of its contextual embedding. Thus, evidence for a greater or lesser degree of idiomaticity, expressiveness and motivation comes from certain types of deviations from the canonical form of a phraseological unit on the one hand. On the other hand, contextual elements and structures that are related to that phraseological unit on the level of literal or figurative meaning of its components or of the entire unit, to the level of metaphor or to the phraseological unit's internal phrase structure provide the basis for systematic insights. The present paper illustrates the practical application of the proposed method by means of three case studies. The contextual behaviour of the German expressions etw. aus dem Ärmel schütteln ('to do sth. with great ease'), etw. mit der Muttermilch einsaugen/aufsaugen ('to learn sth. very early in life') and auf allen Hochzeiten tanzen (similar to the English expression to have a finger in every pie) is analysed in detail on the basis of their occurrences in the 949 mio word-corpus of the Berlin-Brandenburg Academy of Sciences and the Humanities. As a result, the case studies show that idiomaticity, expressiveness and motivation can be quantified on the basis of the criteria proposed here. Nevertheless, no direct quantification is possible, since the evidence has to be interpreted in terms of its function for each individual idiom under analysis.