I have been offered two used amd mi50 but the (re)seller is not certain what type of
power socket pins his cards have. Could be dell, hpe or standard atx pcie. How does
one go about unambiguously identifying the type of power sockets?
They all use standards pin-outs - There are 2 standards PCIe and EPS-12V connectors.
(The standard specifies how much voltage, amp, wattage can be passed through each pin on each standard - No1 is violating that, it would be big mess of dead gpu's if that was the case.)
The first 2 pins can be used to identify which one it is.
If first one is hot, and 2nd is square-ish then its EPS-12V.
If first one is ground, and 2nd has same shape as first, is a sense cable then it is PCIe.
As mentioned NV Tesla series cards use single 8pin EPS-12V.
MI60 uses 2x pcie 8pin's, (so MI50 100% uses same connectors - they are same gpu just cut down a bit.)
You won't find any GPU not using those
(except newest from NV that do 12pin but you won't find it anywhere else)
(and some cards in past could use molex)
In terms of power typically 8pin EPS-12V = 2x 8pin PCIe power connectors. (and vice versa)