How to know the re-formatted book is centered?

Question

I have a tex file that I'd like to print into a paperback book using lulu.com. It is formatted into A4 format and I have done some fiddling with the formatting to get it into the format of a small paperback (the "standard" Dover Books format).

But I'm not sure it's all still centered. Also I'm not sure whether it was centered to begin with since one might give it more space where the page will be glued.

Question 1: How can I ensure that it's centered?

Question 2: Does it have to be centered or should it have more space on one side to glue it?

To provide some more information: I changed the following settings

\def\pageheight{24.01truecm} 
\def\pagewidth{18.12truecm} 
\lhoffset=-0.97truecm
\rhoffset=-0.63truecm

into

\def\pageheight{18.30truecm}  
\def\pagewidth{13.9truecm} 
\lhoffset=1.5truecm
\rhoffset=1.84truecm

Answer 1

There is a good introduction how to design a book's page at the beginning of Markus Kohms "KOMA-script" manual, including the "binding correction" (in German: Bindekorrektur), option "BCOR". If you calculate without it, the inner margin of your book might become so narrow that you can't read the lines.

It makes no sense to place the text exactly in the middle of the page, please read what Markus wrote. You should get the manual with the command texdoc komascript.