This will most certainly vary based on the model#... some devices do not support USB OTG, and cannot function as a USB host and will NOT provide regulated 5V out of the micro-USB charging port.
Additionally... Android is one of those things where every manufacturer likes to add their own 'twist' to the software.
Android should automatically supply 5V when a USB client is connected to the device with a USB-OTG adaptor.
There is a high probability that there must be a USB device present for the USB port to provide any useful amount of current on the 5V line.
... May I ask what you are trying to do? Using a cell phone to power something probably won't work nearly as well as buying a cheap USB battery backup... they are a fraction of the cost, easily put out 5V for extended periods of time, and are easily replaced if you damage something
Then you should purchase a USB-OTG adaptor and check to make sure your device supports it.
Yeah... that's about the only thing I can think of as well... rely on an external supply.
I don't think you can 'force' your device to output 5V... It should do so automatically to any attached device (Like a USB mass storage device)
That said... phones are not meant to output 5V... USB-OTG is a recent advancement and you can expect your battery life to suffer horribly
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