Look at the Market Depth. On the E-Trade site, (and I presume the other broking sites have it) bring up the company code, and check "market depth" This will give you a snap shot of that particular moment in time. You will see a line of buyers with their differing bids, and you will see a line of sellers with their various asks. The top bidder, and asker are seperated by an amount. The wider that is (just at this moment in time) the "jumpier" the stock will be on that days trade. If the bid and ask is only say 1c apart, then at that moment in time the stock is likely to see a higher number of trades. (The bidders have almost reached the "reserve") Also look at the differential in asks and bids as you go "down the line" If there are big price gaps, then there would be a higher chance of a more dissapointing sale price for your sell order. (at that particular moment in time)
The market volume (daily) is the safest refuge. The less daily volume, the more dangerous. A higher volume per day will mean that there is less likelyhood of finding no buyer for your stock when you think it is heading south.
Cheers
Rod