Here are the options.
1. There is word of mouth (or forums).
2. Some imprecise set of numbers, that can't really capture everything or even all that much about a rubber. Right now we have "speed", "spin" and "control".
3. Or you can try it yourself.
That is all there is. The third option is the best, but expensive unless you just try a friend's blade.
Forum descriptions are sometimes vague and not always very correct, but convey more than the three numbers we get now especially when several people say the same thing. Still far from ideal, I get that. (Even Dan and Tom's video reviews are sometimes hard to evaluate because they always seem to like everything!) Adding one or two more numbers to what we have now isn't going to help much, but good standardized graphs might be ok if the vendors don't double prices in response to a new ITTF mandate (like happened after the speed glue ban and with 40+ balls), and if they cut across manufacturers.
Buyer beware. If you insist on trying to find equipment solutions to weaknesses in your game, you may spend a lot and never find anything but minor variations of what you started with. Manufacturers are counting on this. I have wasted lots of money this way and I know better, but it comes down to I don't mind spending money on my hobby and I like new toys. Good thing I don't race cars or fly planes.