"resistance of preamplifiers"? Do you mean input resistance? Are these preamps for moving coil cartridges or something?
In your case it's probably easiest to apply a known voltage and measure the current, or supply a voltage through a resistor and measure both current and voltage. Here is an easy example.
Assume your preamp has a DC input resistance of no more than 100 ohms. It must be DC coupled, i.e. no DC blocking capacitor in the input path. If it's for a microphone or a cartridge it probably won't have a capacitor so you'll be OK.
Let's say we want to put 100 mV maximum into the input, and it has a resistance of 100 ohms maximum. Ohm's law says I = V / R, where I is current in amps, V is voltage in volts, and R is resistance in ohms. This gives a current of 1 mA maximum to prevent damage to the preamp.
Let's say you want to use a 1.5V cell as your power source. An AAA, AA, C or D cell is fine. You need to put a resistor in series with the cell, to limit the current that's fed into the preamp. You need to calculate the value of that resistor using Ohm's law, rearranged into R = V / I where V will be the voltage across the resistor, which is the cell voltage minus the voltage across the preamp input (because the resistor will be in series with the preamp input, so the voltages will add together).
------ 1.5V cell --------------/\/\/\/\/\/\--------------------
| . . . . . . . . . . . . . . . . . resistor . . . . . . . . . . to preamp
-------------------------------------------------------------------
With V=1.4 and I=0.001 (remember, I is in amps, not milliamps), R = 1400 ohms. The closest higher preferred value is 1500 ohms or 1.5 kilohms.
If you make up that circuit using a 1.5K resistor, you will get a voltage across the preamp input, which you can measure with your DMM, and a voltage across the resistor, which you can also measure. From the second measurement, using Ohm's law, you can calculate the current flowing in the resistor, which must be equal to the current flowing in the preamp input, because they're in series. Then you can use Ohm's law again to calculate the resistance of the preamp input.
Ohm was a pretty dull guy, but his law sure does come in handy