the adapter you asked about would split the 8-pin into 2 6-pins. easy to find. you only need 6-pins so just break of the +2 and you're good to go.
https://www.amazon.com/Express-Adapter-Graphics-splitter-TeamProfitcom/dp/B0796NVNY7
as far as a way to check would be to use a power meter and load up the connection. like reviewers do when testing products. in general though, if the psu is capable of providing the power, then it gives the connection. if not, then it leaves it off. super low end junk psu's though don't care and would put 600w worth of connections on a 250w psu cause it looks good and folks will buy it.
what is the EXACT psu model you have? take a pic of the label on the side of the psu if you need to. plus what are the rest of the system specs? all this together, will tell me if it's a good idea or not. once i know what kind of power draw you already have vs the psu's capabilities.