How to know when it is subjunctive or not [duplicate]

God blesses America.

This is the Indicative Mood. It is just a statement that God (regularly) blesses America. The third person singular takes the inflection -(e)s.

God bless America!

This is the Subjunctive Mood. Here you are not stating anything. Instead, you're expressing a wish. The third person of the subjunctive coincides with the infinitive. Other examples of the subjunctive

God save the Queen!

Be that as it may, ...

It is important that he be here on time.

By saying "God Bless America" you are indicating a desire that God will bless America in the future. You could even term this a prayer.

"God blesses America" would be an indication that you think the blessing has already occured - or that the blessing is currently occuring.