Some Indications that you can observe in your computer after infected with malware:
1. Slower internet connection and processing.
2. Unexpected computer behavior like auto open of files, no response to the clicks, ect
3. Sudden freeze of computer.
4. Error messages regarding missing of some important files.
5. Frequent advertisements even when you not active in browser.
these are some important observation that you can see to identify the system is infected or not.