When to test?

Hi all

I last had af 20th Sept, would of been sometime late last week, dtd twice during the week, and now had brown spotting last night and early this morning nothing since, normally when I have my period I come on heavy red blood straight away. So I'm thinking maybe implantation bleeding. I just wondered when's the earliest I can test as I'm impatient? And other people's experiences with implantation bleeding... would the timing be right and does it sound similar?