Well, for an objective metrics, benchmark is always a good way to know for sure.
For an all-rounder benchmark that tests I/O and connectivity to several areas, run this on command line:
wget freevps.us/downloads/bench.sh -O - -o /dev/null|bash
I'd say that if the IO you get is below 50MB/s, it's probably not a very good/oversold VPS. If you are promised 1gbps port, see if connectivity at least reach close to that point. For a more accurate benchmark of IO, you can use Bonnie++ as well.
For cpu benchmark, there are plethora of options but i usually use geekbench 4, of which there is a bunch of guide online on how to use, others use unixbench. CPU benchmark is used to see how much throttling is in place (if any).
: Forgot to add, for routing, you can utilized several looking glass from and to the VPS, to see what kind of routing is being used. I usually use BGP and MTR for this.
: Whoops, i didn't realize OP was asking for a pre-order signal. Well, best way to know for sure is ask for a test IP/looking glass, and identify the ASN using IP to ASN converter to see who actually owns the IP space, check the routing, see if they have moneyback guarantee and use the benchmark option above. Hope that's useful!