How does Google knows i use Redirect(301/302)

Hello,

there is something about redirects that i wish to understand,

lets take an example case,

i have the following pages:
Category A with 5 pages on it,
Category B with 5 pages on it,

category B is 90% similar to category A in terms of content, hence its duplicated content
so i want to redirect category B page to category A page, and all the 5 pages under category B to their matched page under category A.

i do all the redirects (301) and now there is no more category B under my site and all the link juice that category B (and the page under it) are transferred to category A and its pages.

all good till now.

now here the tricky part (the question),
there is no links referring to category B or its pages in my website or any other external website. so Google/the crawlers will never see the category B links anymore so how will they know it has been redirected (301) to the new location and give it the link juice?
- i know it does follow it and finds it, and very quickly also in terms of SEO time. (usually faster the the crawler will meet your site again).

what i was thinking is:
- the crawler got the page url indexed in his memory,
- the crawler do some large scale scan for the indexed urls he got for responding headers status, and everything that is "changed" he will update and take the appropriate actions (aka giving link juice in this case).

will be glad to hear your toughs / facts about this issue,

thanks in advance,

p.s - sorry about my English, its not my native language.