How to find polarity of terminals of a circuit using meter?
If you post a picture or two of your PCB (close ups of input and surrounding components) it would be easier to help confirm things.

Often, there will be some indication on the PCB silkscreen at the power inputs of polarity. If not you can often tell by following the input trace(s) and seeing what it connects to. For example, if the input pad is connected directly to a large copper fill (you may see 2 or 4 "wheel spoke" type connections to the fill), then it's almost certainly ground (not always though).
You can also look for things like power filter capacitors and see if the negative side (usually a stripe on that side probably also polarity symbols)) is connected to the input pad (i.e. has 0 ohms on multimeter ohms range, or beeps on continuity setting)



Similarly, if there are any ICs on the PCB you can get datasheets for, look at the pinouts, see which pin is ground and test between this and the input pad.



For the positive input, you would look for the input to the other side of a filter cap, connection to an IC regulator input (example datasheet from which above pinout was taken), any LEDs (on a typical 5mm LED the flat side is the cathode or negative side, so the other pin may be connected to the positive rail, probably through a small value resistor - it's cathode may be connected directly to ground)