I'm not sure if there is a simple way, as such. Assuming that you know what inflectional and lexical suffixes are and how they differ, you can usually tell simply by looking at a word. Inflectional suffixes are used to change the properties of a word to express a different tense, mood, voice, or other grammatical category.
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ssharboHow can I determine the difference between a inflectional suffix between a lexical suffix?Memorize all the inflectional suffixes. All the rest are lexical suffixes.