Adjective: a word that describes/qualifies noun or pronoun.
He is a good boy.
Here, in the above sentence good is an adjective which describes the noun boy.
Past Participle: it is considered to be the third form of verb.
You need to perceive the usage of past participle by learning more about English grammar. If you have knowledge about grammar, then you can easily learn to mechanize and distinguish between an adjective and a past participle.
Here is an example, which I hope will illustrate my meaning.
He has demolished his house to reconstruct it.
The above sentence is in the present perfect. We already know about that in present perfect or perfect tense of other tenses i.e future or past have the mechanism of past participial form of verb. So from here we get the past participle form which is always preceded by the helping verb. In the above mentioned sentence, demolished is used as a past participle form of the verb demolish. We come to know after that it is preceded by a helping verb.
Usage of irregular adjective: adjectives which end with the addition of ing or ed forms. They are mainly called irregular adjectives.
Point to Ponder: keep in mind adjectives must precede the noun or pronoun.
An interesting match
The interested candidates
These are irregular adjectives with the same meaning of the noun precedent.