Is there a standard that specifies the polarity when using an SMA connector (or similar) to connect a DC power source?
I'd expect the outer conductor to be the one that's on ground potential, for practical reasons; I'd expect the bias voltage to be on the center conductor. However, whether that voltage is positive or negative to ground would be defined by the application. So, if you have an LNA that needs positive bias relative to ground, then that center conductor would be positive relative to the outer conductor, and vice versa. 

Don't expect a standard; I'd agree, I'd expect a higher probability of the center conductor being positive relative to the outer, but you'll always have to check.