How to recognize an OTG USB port on an embedded device?
There are no OTG Type-A ports, simply because the 4-pin Type-A connector is lacking the extra ID pin that is necessary to switch port roles from "device" to "host" or vice versa. If some goofy product has a Type-A port that is used to re-flash firmware, this is just a wrong use of "OTG" terminology and misuse of USB specifications. Most likely this port is pre-configured as "device", and it shouldn't be using the receptacle of Type-A. 

If my guess about the Rockchip-based devices is correct, their "OTG port" would be the one that doesn't have VBUS source. Therefore USB flash drives won't work in that port.  You can either try to measure VBUS voltage on empty receptacles, or just try your USB flash. The port that doesn't have VBUS or flash fails is your "OTG" port. You will need a Type-A &lt;-> Type-A cable (again, illegal from USB standpoint) to reprogram your device. Keep in mind that A-A cables might have D+/D- swapped, so don't be surprised if some (illegal) cables won't work for you.