How to find out how much voltage and current a soldering agent can handle
Solder isn't wire.
Unless you are dealing with extreme voltages and currents, you shouldn't have to consider the conductivity of the solder at all.
The joints you make with solder are comparatively small.  The resistance of such small volumes of solder isn't usually high enough to consider.

Do not try to use solder as wire.  It is not made as a replacement for copper wire.  Solder is only for joining things together.