There is no way to know for sure without a chronograph. I've found that in most cases I get pretty close to the advertised velocity, sometimes a little faster. The key is barrel length. If your gun has the same barrel length as the one used to test you will be pretty close most of the time. If the ammo was tested in a 24" barrel and your gun has a 22" barrel you might see 50 fps difference.
I do see differences with individual rifles, and some brands of ammo. I've seen as much as 130 fps difference between different rifles with the same length barrels with ammo from the same box. With most brands of ammo the velocity proves to be almost exactly as advertised in my guns, with some others I've seen it run 100+ fps slower than advertised.