I'm not sure that I've ever seen a multicore solder for other than electronic use.
However, the places I go only sell solder for electronics use
Even if my first observation was correct, you would at least want to know if it is a tin/lead or a lead-free solder because they behave quite differently.
What is the diameter of the solder. If it's 1mm or less, it's unlikely to be for anything other than electronics. That answers the main question even if it leaves others unanswered.
One thing you can do is to melt a puddle of the solder. Have a blob about 6mm wide and melted. Then get a screwdriver and poke i around as it cools.
one of three things will happen (for leaded solder):
1) As it cools it will get "pastey" and you'll be able to shape it while soft and even smear it out.
2) It will go from liquid to a solid almost instantly (say, over the period of a second) at some point
3) It will go very quickly from a liquid to a solid at some point, but you will be able to mush it around for a couple of seconds.
(1) is for soldering downpipes. It may be 50/50 or even further away from the ideal.
(2) is 63/37 - right at the eutectic point for lead and tin. This is very good for electronic work
(3) is 60/40, quite typical for electronics work.
Regardless of what you have, if it is thin, my prediction is that it will behave like (2) or (3).
If you have a temperature controlled soldering iron you should be able to estimate the melting point. That will tell you if it's leaded or lead-free (which almost always have a significantly higher melting point).
Incidentally, solder melts like it cools. If you can estimate the range of temperatures over which it remains between solid and liquid (it's a bit subjective) then you can estimate (or maybe postulate) the composition.