Out of curiosity, what's a good way of knowing how many mA a piece of hardware is going to pull?
There are two ways, basically - one is to consult the datasheet for the device in question. If that isn't available, and the device is fairly basic (like a motor), then direct measurement of the current with a multimeter can be done.
I'm assuming smeezekitty's assumption that the motor would be more than 40mA was based upon some pre-existing knowledge--but is there a basic cheat sheet or rule of thumb?
Not really anything that would work in all situations...
Does the motor shield make this sort of simple interaction (Arduino to pager motor) easier? From what I can tell from the FAQ it's designed more for larger 6v+ motors.
As always, consult a datasheet:http://www.datasheetcatalog.org/datasheet/texasinstruments/l293.pdf
Here, you'll see the L293 can run motors from 4.5 VDC to 36 VDC, up to 1 amp continuous current draw. You'll also notice its a dual-bridge, so you can run two motors with full control of direction and speed. As you approach the top-end current draw, though, you would want to incorporate a heatsink on the chip, which the datasheet describes.
Yeah; anything using an L293 is best for small motors which run on 5-6 volts, 1
I noticed the specs have "4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, internal kickback protection diodes. Can run motors on 4.5VDC to 36VDC."
Unfortunately, this bridge likely won't work with your motor, as you motor is likely a 1.5-3.0 VDC motor; while you could "overvolt" the motor with 4.5-5.0 VDC, its bearings and other parts will likely fail in short order, leaving you with a burned out motor.
Your best best for this kind of motor would be to build your own discrete transistor h-bridge, made from NPN and PNP transistors; 2n2222 (NPN) and 2n2907 (PNP) transistors are complements of each other, and likely perfect for the job - here's an example circuit of such a bridge:http://www.solarbotics.net/library/circuits/driver_4varHbridge.html
You can build it in a very tight back-to-back transistor mounting fashion - as shown on the page.
Finally - on measuring the current of a motor:
For small motors (which you are fairly certain pull under 10 amps), putting a multimeter in series with the motor can work fine. Put the meter on current measurement for 10 amps, put the positive probe in the current measurement jack (some meters have a separate jack for 10A measurement; if you know your motor is likely under the smaller amperage measurement, you can leave it there, but it is generally best to start high, then work low - keeps you from blowing your meter).
Hook the meter in series with your motor, then verify everything (including the meter setting!). Then, hook it up to your power supply or battery, and run the motor. Note the reading of the amperage with the voltage. This is the running no-load current.
If your motor is small enough (don't try this on really large motors), grab the shaft to slow it down, or even stop it (briefly!) and note the current reading for the voltage; it should be much greater. This is the stall current.
Your h-bridge should be sized to handle the stall current, at least briefly; its design should allow for continuous running capability of the motor at a current about 50-70% of the amount between the running current and stall current. For example, if your running current is 10mA, and the stall current is 100mA, then the h-bridge should allow for up to 100mA intermittently, but have a continuous rating of around 45-65mA.
For larger motors (>10 amp current draws), the best way to measure is via a very small value shunt resistor (this can be something simple like a small length of thick copper wire). Knowing the resistance of copper wire for its cross section (gauge) and length, you can measure the voltage drop across the wire using you meter, then apply Ohm's Law to calculate the resulting current being used.