By law, Hubpages and other sources like this sends out a statement at the end of the year. I forget the number used on the form, 1099, I think. This gives your earnings for the year. The cut off for these notices is either $400 or $600. In which case you do need to know how much you earn so you can file taxes on that amount. You do not need to file taxes if you earn less than $400. I think that is the cut off. But it may not be anymore. So check the IRS. :)
I hope this helps.