How do I know 360 Security antivirus app really works?
First, That app is shit ! 
Second, It does nothing but clearing junk and cache of the apps on your device.  
It will always prompt you with ads and unwanted bullshit. 
There is no such thing as virus in your phone. If it were then it will make you almost impossible to operate your phone as the malware and torjans designed these days are pretty cool.

 Clearing app junk and cache is now a built-in feature in android. 
The memory booster does nothing but killing all the running apps in the background. Friend! My advice is to un-install that app ASAP !! save some memory than having an useless app