Good question. The answer is that irregular means 'not following the rules'.
In this case, the "rules" are for the regular verbs, and they say
- The past form and the participle form of regular verbs are the same
(i.e, "2nd form" = "3rd form")
- These forms of regular verbs add "-ed"
(sometimes spelt differently).
So irregular verbs must violate those rules, which means either
- they don't use "-ed", or
- they have three forms instead of two.
Any dictionary will give the forms of a verb, but only in code. Here's the key:
- if the dictionary indicates that the past form is "-ed", or "-d" or "-t",
or if it's a special form like spent, where "d" changes to t,
or if there is no participle form given,
then it's regular
- if the dictionary gives three forms (like sing, sang, sung),
or no verb form ends in "d" or "t" (like wear, wore, worn),
then it's irregular.
Plus, there are very few irregular verbs, and they follow common patterns, and they occur often.
Which means you can learn them from a list, like this one.