The following assumes the camera is a DSLR.
If you can see dust while looking down the lens then, unless you have moved the mirror up, it is not on the sensor, which is good news.
It could be within the viewfinder or on the prism/mirror. Even if the lens is new the dust could still be there. It is easy for dust to get just about anywhere, but if it does not show up on your photos it might not be worth bothering about.
Dave