How do employers find out about your employment history even you didnt included it on your resume?

I just need an answer to my question on the title above. Idk if this is the right sub but I need help for this because I am so confused, worried, and shit at the same time. So I submitted every government numbers for my pre employment requirements on my new employer (BPO) thru email and signed the JO. They will conduct a background check on me before the training (not sure when will it start). In my resume, I didn't include the 2 previous companies because of AWOL (only 1 month on each) . Now I read somewhere that the employers can check your employment history on your SSS, Pag-ibig, and Phil health and has a higher chance that you can't proceed on your employment. I still wonder how will they find out about your employment history.