First of all, rocks from Earth are probably just about everywhere in the Solar System. One simple example is this rock found on the Moon. A number of pieces of Mars have been found on Earth, and if that has happened, no doubt there are Earth rocks on Mars. If Earth life can survive a vacuum it has probably gotten to everywhere in the Solar System anyways. Lots of precautions are taken to minimize the chance of spreading life elsewhere, but we can't say for certain that we haven't accidentally sent life. The important thing is to keep the chance to be less than that of traveling via space rocks.
For Planetary Protection measures now, which help to prevent spreading life to other planets, that is taken in to account. There are a number of categories for planetary protection, from Category 1 which isn't thought to have life at all, to Category 4 where we thing that life is possible, Mars, Europa, and Enceladus being the 3 objects now there. Venusian life, if it exists, will be so different from Earth life that we aren't likely to spread any life there.
There are a set of actions that are required for each level. Landers require more protection than orbiters. Landers that are wanting to detect life have higher requirements than other types of landers.
With the exception of Mars, and to a lesser extend Europa, most of this is just theoretical. For Mars, the idea is to build the spacecraft in a clean environment, basically a clean room, with some sterilization. Components that can tolerate it are subject to high heat. Electronics are kept in a mostly sealed chamber, vented its atmosphere to space through a filter to keep bacteria in. The booster that launches it to the planet can't land on Mars itself, the spacecraft must control itself to get to Mars on its own, although the amount of thrust to move away is quite small.
For landers on Mars and likely Europa, there have been varying standards throughout the years. The Viking landers were heavily sterilized, but they were intending to detect life, so that was required. Some of the instruments on other landers have been similarly sterilized, like the Phoenix Lander arm, which was sterilized and only released from that sterilization chamber after landing.
For the most part, most areas on Mars aren't considered to be able to support Earth life. The radiation is believe to kill any bacteria that will fall from the spacecraft to the ground in that period of time. Furthermore, any life that could survive that trip has likely already been sent to Mars anyways. There are a few parts of Mars that are considered more sensitive, those areas that might have liquid water. Curiosity landed near one of these, but was somewhat limited in its ability to explore there because of the lack of planetary protection measures.
Any Europa lander will likely take the highest levels, although the radiation environment on the surface there is extreme, any bacteria won't survive. Especially if a spacecraft lands beneath the surface then things are particularly important to pay attention to this, and thus an ultra-sterlized spacecraft will be required.
There are some other things that are done as well, like ensuring that all parts of the spacecraft are pointed towards the sun for a period of time to bake out any bacteria, but I haven't found any sources saying how much this is done. But the bottom line is, we take reasonable precautions to ensure that we don't spread life to other places in the solar system beyond what might have naturally occurred.
