必勝 is on the flag, and yes it means certain victory. There are also the words 寿 and 吉, both which mean "good luck" more or less.
These random words do nothing to make this flag into an authentic WW2 item. In fact, they feel kind of weak. It's like if I posted an American flag with the word "fight" written on it and claimed it was a WW2 item because of this word... It really doesn't mean anything. Usually you have one or two very prominent patriotic phrases, and ideally you have the name of the giver and of the receiver. From there you can take a guess at authenticity, but even then it is often just a guess. One thing that makes flags look suspicious is a bunch of names written with the same brush. Or names that use characters that aren't typically used in Japan. Or flags with weird symmetry in the names.
The flag you posted is just full of names, and one or two random words. If you are looking for a Japanese flag with names on it, this is a good one. If you are looking for an authentic bit of WW2 militaria, its hard to recommend this one.