Here you will find some useful suggestions and guidance on How to Find a Bug in Application. Also, understand the different approaches to defect discovery:
Testing is often simplified as a bug-finding activity. Even though that’s not the entire job profile of testers, finding problems before the end users do is the long and short of our core duties. So, should testers worry when they test software and find no defects?
In other words, do zero defects mean no testing occurred? Not true. All testing finds defects. All software has bugs. It is up to the tester’s skill, time available, background information provided, domain knowledge, resources allocated, etc. that influences the question – How to find defects in an application?
It is hard to quantify how to develop the skill of finding defects but be assured that testers get especially better with experience and exposure to various kinds of software systems and their functionality.