It should say on the container if it is flux core. If it doesn't, then it probably isn't. You will know as soon as you try to solder something. If it just clumps up and doesn't want to flow smoothly, then chances are it has no flux. And as was previously stated, you should be able to see the core if you look at the end of the solder.
try soldering something besides your megadrum to get a little practice and work out these kind of bugs.