Is perfidious an adjective in this sentence ?: Tom's wife is perfidious.

Yes. It's the predicate adjective:

http://grammar.ccc.commnet.edu/grammar/objects.htm#predicate
Scroll down to the section on Predicate Adjectives.

i need to put outwit in a sentence

Yes, "perfidious" is indeed an adjective in the given sentence. It describes the quality or characteristic of Tom's wife.

To determine if a word is an adjective in a sentence, you can follow these steps:
1. Identify the word in question. In this case, the word is "perfidious."
2. Determine the part of speech of the word. In this case, "perfidious" is an adjective because it is describing or modifying the noun "wife."
3. Confirm that the word is providing additional information about the noun or pronoun in the sentence. In the given sentence, "perfidious" is describing the quality or characteristic of Tom's wife.

By following these steps, you can identify the part of speech of a word in a sentence.