Given two or more satellite position sources, how can one determine the best one to use, or how should they be combined?

HDOP: This error depends on the geometry of the satellites. It's the same idea as having good angles of cut when taking bearings. It will be the same for all receivers then, except that ones with a more sensitive receiver / better aerial might see more satellites and therefore practically get a better HDOP.

xxGSA contains HDOP.

uBlox uBlox M9 Interface manual section 2.5.5 NMEA position fix flags shows output for various situations.

posMode flag descriptions in here: https://content.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_UBX-13003221.pdf.

Some of the uBlox receivers have dead reckoning capability and might output DR positions. quality and posmode fields indicate if there is a fix or DR.

$xxGSA,opMode,navMode{,svid},PDOP,HDOP,VDOP,systemId*cs\r\n

GLL / RMC Status: A for valid, V for invalid. GGA quality