1. Check whether the language is finite or not. If its finite then it is surely regular.
2. If its not finite then there may be a case that its not regular.
3. Rules for checking if regular:
a) Check if there is any pattern in AP then its regular.
b) If there is any irregular pattern like in GP or in terms of prime then not regular.
c) If automaton requires saving of infinite length string then comparing then its not regular.
d) Modular counting or modular division is always regular.
4) Pumping Lemma is negativity test means
a) If passed then language may or may not be regular.
b) If failed then language is definitely not regular.