Detecting power along a pair of wires

I apologize for the electronics 101 question. I've been reading the forums and this group seems friendly enough.

My specific project I'd like to consider is creating an arduino system to monitor an older vending machine (no MDB or DEX) and sending the results via cell phone. I've found examples of sending with a cell phone and I'm not worried about that.

I figure what will probably be the only way to do this is to connect electronic sensors to the wires leading to each vending machine motor and detect when the power is given to each motor as a way to detect when that product has been vended. Seems simpler than doing something to detect the motion of the vending machine coils. So my two questions at this point are:

1 what is used to detect the power? I think this is some kind of transistor - one part of the transistor is hooked to the power, the next to the ground of the vending machine and the third to an input pin on the arduino board. Is this correct and which type of transistor would I use? Seems wrong to me but I never understood transistors.

2 Using binary logic I should be able to take 50 inputs and reduce that down to 6 inputs. My basic idea would be that if there was power on wire 1 that input 1 would be triggered, wire 2 would trigger input 2, wire 3 trigger both input 1 and 2, etc. Is this the best way? I'm assuming there is some kind of chip I could use but I have no idea what one to get. Any suggestions?

I've read some basic electronic books in my younger years and I can follow schematics & solder but I've never designed anything and I don't want to read a full book if I can avoid it. I've done some forum searches and not found what I needed.

Thanks in advance.

To answer your questions Richard:

1 Well right now I have two vending machines out there but I'd theoretically build one for each vending machine I do. I'd have one unit in each vending machine.

2 Lets use a full size snack machine as the example so 45 motors.

3 I'd like to in theory know exactly how many of each product was delivered so I'd know what to bring when I stock the machine. I've programmed in PHP and Python so I think I'll be able to figure out programming the arduino to store the data (yes I know this is low level programming but I'll figure it out.)

4 There will be other loads in the machines but in theory I'll just be connected to the actual vending motor. These motors have only two wires. I believe they work by having the power start from the controller board then they shut themselves off after rotating 360 degrees. I don't believe they have power along those wires when the motor is not operating but I'll test that in the future.

5 - I believe 12 volts but I'm not 100%.

6 - Approximately 5-6 seconds. In the software I'd program a delay or some way to detect the power turning back off to avoid it detecting one motor rotation as 1,000.

Awesome. Thanks for pointing me in the right direction. I'll do more google searches to learn how to use these.