How to know if my batteries are genuine and not counterfeited ones?

But what about the bios and ePSA test part?

BIOS and ePSA do not read serial numbers or validate if the battery is Genuine. As dell allows their customer to install third party parts. 

They will do first test if the signals going in and out from battery connectors on the motherboard to battery. If not ePSA will throw error 2000-0131

They will do the second test checking battery overall remaining capacity. (mAH) Depending on remaining capacity BIOS will call the battery (Excellent/good/Fair/Poor) 

Based on the same capacity ePSA test will either pass the test or give you error code. 2000-0133. 

Unless you are saving a lot of money do not buy from third party seller. Call dell and atleast take a Quote for battery 

dell identified their parts by PPID numbers on them. Sellers with fake batteries can copy these numbers on their labels. Dell will not provide warranty support on these batteries no matter how reputed the seller is. 

All in all there is no way for Dell or you to check if the battery is original Dell battery. Dell provides warranty support only if the battery is purchased from them. 

Edit : Dell charges minimum 160$ for a new battery .