The best way that I know of can be found here http://www.elitetrader.com/vb/showthread.php?s=&threadid=15110 Just in case the link dose not work,type in "Keeping it simple II" in the search box.
I use the pattern of the overnight market as a key. I also use the most recent overnight hi +0.50 and low -0.50 as buy and sell stops on the opening. On mornings like today - there was an hour of narrow congestion before the open (SP) so i used once the market opened above the congestion, I used the congestion low -0.50 as a sell point getting short at 988.20 (ES 988.25) and a reverse stop 0.50 above the high at 990.00. However, I do not look at "trend days". I am only looking for a profitable move, my system uses an initial set stop and set exit, then adjusts stop and exit as trade progresses.
I think this is one of the toughest tasks in trading. One reason is there really are not very many true trend down days over thecourse of a year. The big down days tend to retrace a lot, while it is more common to have up days that never look back. Just my opinion of ocurse, as I don't have any stats to back it up.
This is a great question. How to put this day into some number pattern or mathematical measurement is not for me. I'll bet someone here can come up with it though to recognize it early. For me it simply felt like it. I think this comes from just watching the market day in and day out for a few years. Today was a classic day. Methodical declines with small shallow pullbacks to the 20sma on a 1m or 3m chart until lunch where it corrected and then sold off again into the afternoon. The late day surge was kind of atypical for this trend down day but was great and the only area I made some $ today.
paulus, it is not necessary as a trader to guess which way the market will go. don't bother yourself with trying to predict the future. focus on developing a system that works and follow that system religiously. good luck.