If brandy is marked kosher for Passover by a reliable certifying agency, then you can rest assured that it is just that. After all, the base ingredient of brandy is fruit, which is kosher for Passover. However, since spices and other flavors are often added to the mix, you cannot be sure that brandy (even if it is marked kosher) is kosher for Passover, unless it bears Passover certification.

It is important to note that, as you can read here, some (including Chabad) traditionally avoid all alcohol on Passover (with the exception of wine).

Also note that since brandy can contain many ingredients (including wine), and factory conditions may vary, all brandy must be certified kosher, even for year-round use.