How do they know when God is angry?

[Believe Heart]

nope they can not.
They can't be right and wrong,
They can't be negro and white at the same time.
Your beliefs can be true or false there is no midway between them.

1. Actually they can be right and wrong. Some of their beliefs could be correct and the rest could nonsense 
2. Negro is an invented term it has no objective meaning and has been applied to whites in the past 
3. A set of beliefs can be partially right and partially wrong. Jesus can exist but not be the son of god even if he completely believes he is and all his followers were willing to die for that belief

Nope that is also not possible.
Believe means "God". God means Perfection. Therefore anything about a religion which is doubtful is not acceptable. Therefore I, for example even don't accept lots of phrases in Hadith books, because they look suspicious to me. 
I only accept and believe absolute truths.
That you can find by scientific methods or you can get from a real divine message.