Estimating a bug is a very easy job for a person who is having good exp and good domain knowledge and at the same time if tester knows about the developer (strenghts and week ness) then it's an added advantage.
As above said we can estimate the bugs with past experience and Domain knowledge.
if you have good knowledge on Languages (vb.net java etc) Databases OS browsers etc. you can estimate the bugs too as you will have idea about the advantages and disadvantages of the above langauges or DB etc.