It is really easy to check on the ranking of any Kindle book. Go to the page of the book you want to check and scroll down until you see a heading "Kindle rankings". Depending on the categories which have been assigned to the book, I think by Amazon but unsure on this, you will see your book's current position in the best selling list for that category. In the updates on this thread I always use and quote the Alternative History category.
If your book is a new release then, depending on sales, it may make it into the top 100 listing of new releases - this categorisation lasts for up to 30 days.
To check on this, click into the category listing shown on your book's page which will give you the top 100 best sellers. Scroll down this page and you will find a link to the top 100 new releases.