You can check that the Sender ID is gov.sg, to identify that it is a legitimate SMS sent by a Government agency.

All SMSes from the government will be within the same gov.sg Sender ID SMS conversation. Additionally, each SMS sent from the gov.sg Sender ID will include a header indicating the name of the government agency or service, from which the message is being sent. Recipients should also practise caution with messages sent from similar-looking Sender IDs that may use slight modifications (homoglyphs) in their Sender IDs.