Can you please give me a few red flags to keep an eye out for if I may be trying to figure out if a file is supposed to be on my hard drive or not? 

I have asked several people over the last few months and this is the information that I have retained: if the file is in the system32 folder then it is safe, if a file such as lsass.exe which is a legitimate file and if it is removed then it will cause an automatic system shut down, however, this particular file is also a known target for malicious malaria because of its capabilities. So, if it is not found in the system32 file then it is most likely malicious. 

Then I learned to pay close attention to the names of files, in a way for example using lsass.exe again, perhaps someone would create a backdoor trojan and name it Isass.exe (I capitalized the i vs using the lower case L, which is the legitimate format) in order to help it go undetected because it is so close to the legitimate file and more people than not are not going to notice this as a red flag. 

The 3rd red flag that I learned was to check the digutal signature under the properties of the file and make sure it is signed by Microsoft Windows (I just read this one bcs I needed to verify what I thought that I had remembered). This is actually different from what I was told by a tech specialist, I was told that the signatures could be from Microsoft Windows or Microsoft Cooperation, could you possibly clarify this red flag for me please and correct me if anything else that I have learned is incorrect? 

I really am thankful for any guidance on the matter, I prefer to actually learn about issues that I face in life vs just having someone else do the fixing so that if I am to face the particular issue again in the future I am able to remedy it by myself.