Actually, you can see their method of attack... they stand out like a sort thumb.
Take this file which is a recent sample I did just about an hour ago...
http://www.splitinfinity.com/brutes.txt
Scan through that.... can you identify the brute forcers? :-) Easy.
Look for sections that look like this:
204.186.159.229 anthonym
204.186.159.229 army100
204.186.159.229 bluepoint
204.186.159.229 cuco
204.186.159.229 EDOG12
204.186.159.229 fibbsjc
204.186.159.229 forxxxhq
204.186.159.229 gdromey
204.186.159.229 joe412
204.186.159.229 judges
204.186.159.229 laura
204.186.159.229 n6tb6x
204.186.159.229 pikedirk
204.186.159.229 tammygranville
Or better equipped brute forcers do it like this:
207.200.116.12 grimlok
207.200.116.12 grimlok
207.200.116.12 grimlok
207.200.116.12 grimlok
207.200.116.130 grimlok
207.200.116.131 grimlok
207.200.116.131 grimlok
207.200.116.131 grimlok
207.200.116.131 grimlok
207.200.116.131 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.132 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.133 grimlok
207.200.116.135 grimlok
207.200.116.135 grimlok
207.200.116.137 -
207.200.116.137 grimlok
207.200.116.137 grimlok
207.200.116.137 grimlok
207.200.116.137 grimlok
207.200.116.137 grimlok
207.200.116.138 grimlok
207.200.116.138 grimlok
207.200.116.139 grimlok
207.200.116.139 grimlok
207.200.116.13 grimlok
207.200.116.13 grimlok
207.200.116.13 grimlok
207.200.116.195 grimlok
207.200.116.195 grimlok
207.200.116.196 grimlok
207.200.116.197 grimlok
207.200.116.198 grimlok
207.200.116.198 grimlok
207.200.116.200 grimlok
Note the rolling ip's?