By default, after Spam Assassin processes an incoming message, it will put a mini-report into the headers that looks something like this:
X-Spam-Status: No, hits=3.3 required=7.0
This lists the tests that returned positive. In this case, the CLICK_BELOW and CLICK_HERE_LINK tests matched this message. Their scores add up to the 3.3 listed. Because 3.3 is less than the 7.0 required, this is not likely to be SPAM.
Here is a case where the message was over the required threshold. The subject line will be prefixed with "*****SPAM*****".
In the headers:
X-Spam-Status: Yes, hits=8.5 required=8.0
X-Spam-Checker-Version: SpamAssassin 2.43 (18.104.22.168-2002-10-15-exp)
This will be added to the message itself:
SPAM: -------------------- Start SpamAssassin results ----------------------
SPAM: This mail is probably spam. The original message has been altered
SPAM: so you can recognise or block similar unwanted mail in future.
SPAM: See http://spamassassin.org/tag/ for more details.
SPAM: Content analysis details: (8.50 hits, 8 required)
SPAM: SUB_FREE_OFFER (0.3 points) Subject starts with "Free"
SPAM: FREE_MONEY (-0.1 points) BODY: Free money!
SPAM: WHILE_SUPPLIES (0.3 points) BODY: While Supplies Last
SPAM: EXCUSE_14 (2.0 points) BODY: Tells you how to stop further spam
SPAM: GREAT_OFFER (0.2 points) BODY: Trying to offer you something
SPAM: OFFER_EXPIRE (0.1 points) BODY: Offer Expires
SPAM: WEB_BUGS (2.0 points) BODY: Image tag with an ID code to identify you
SPAM: SPAM_PHRASE_03_05 (1.1 points) BODY: Spam phrases score is 03 to 05 (medium)
SPAM: [score: 3]
SPAM: HTML_FONT_COLOR_YELLOW (0.4 points) BODY: HTML font color is yellow
SPAM: HTML_FONT_COLOR_CYAN (0.4 points) BODY: HTML font color is cyan
SPAM: HTML_FONT_COLOR_GREEN (0.4 points) BODY: HTML font color is green
SPAM: BIG_FONT (0.3 points) BODY: FONT Size +2 and up or 3 and up
SPAM: HTML_FONT_COLOR_GRAY (0.3 points) BODY: HTML font color is gray
SPAM: HTML_FONT_COLOR_RED (0.3 points) BODY: HTML font color is red
SPAM: HTML_FONT_COLOR_BLUE (0.2 points) BODY: HTML font color is blue
SPAM: MIME_LONG_LINE_QP (0.3 points) RAW: Quoted-printable line longer than 76 characters
SPAM: -------------------- End of SpamAssassin results ---------------------
As you can see, there are several things that can be used to filter on. One tactic is to filter anything with the *****SPAM***** in the subject to a seperate folder.