how do I know if message is scam or not

[Petunia22]

Received message  couldn't deduct latest bill from registered payment method.  Visit web address to resolve issue and avoid sim termination. But on my account says bill issued 8th March  payment due 24.

 nothing about an issue with payment.

[pgn]

Never use the link in an e-mail or SMS message to login to your account @Petunia22 - Phishing, Smishing & Scams. Latest info & advice. 

If you have, you will need to change your password immediately and be very attentive to activity on your account over the next few days. Never give anyone the pin you receive via SMS to login to your MyO2 either - the real O2 will never ask for it. Guide: How to find help & contact O2 has the ways to contact O2 if you need to call them. Good luck!