Go to your local automotive store and get an 1156 incandescent bulb. This is used for brake lights and turn signals (typically) and is pretty common. It is 12V and 26.88 watts (about 2.2 amperes). You can connect it in series with one of the wires that goes from the block circuit breaker (or booster if there is not a circuit breaker) to the block bus. If there is a rail to rail short in the block, the light bulb will come on when you turn on the booster. It will limit the current to about 2.2 amperes. Current will flow from the breaker, through the light bulb, down the DCC bus to the short then back to the breaker on the other side of the bus. By limiting the current to 2 amperes, the breaker/booster should not trip, but you will have enough current flowing through the rails to create a measurable voltage drop for troubleshooting. Since you are using the normal DCC, you don't need to worry about damaging anything on the layout. Now use your meter (set on AC) as Don mentioned and look for the lowest voltage along the track. The other thing you can do is measure the voltage from one end of your track drop wires to the other end. The drop line with the HIGHEST voltage is where the current is flowing and is where the short is. The other benefit is that if you start wiggling stuff and the light goes OUT, you just moved (or removed) the thing that caused the short.

One other thing: if the light bulb is bright, you probably have a solid low resistance short, if it is dim, you know you are looking for more of a low resistance than a dead short.