Now a days, WhatsApp is one of the most used social messenger.  you are online on WhatsApp.  At the same time, anyone of your contact or friend or any other person may also be online.  And he check yours whether you are online or offline.  If he or she checks you then it will shown to him or her that you are online.  At the top bar, under your name online is written.

If your last seen is shown to everyone, then when you are offline then the last seen time of yours when you were online will be shown.  If your last seen is shown to nobody, then no one will be able to see your last seen time on WhatsApp.  But when you are online then anyone can see your status as online.

How can I understand that when I am online on WhatsApp someone checked me?

So, Dear Friend, there is not way to check who have checked you on Whatsapp.  Till now, WhatsApp have not release this feature in WhatsApp.  If this feature is provided in near future, then it will be great and we will also update here.

Till writing this article the answer is big NO