There is a lot involved in determining what to do. I'll try to give a brief outline here, but for more information, it is good to research the particular plant in question.
Some plants have perfect flowers (both male and female parts) so they need nothing more than a little wind to pollinate themselves. These kinds often do not cross pollinate at a high rate, most tomatoes and beans for example. To ensure purity, you can bag the blossoms right from the start though since no insect is needed.
You can also separate by distance to ensure purity. Some plants, the distance is not too far especially those that are self pollinating, but may attract some insects like peppers. Having other flowering plants between to act as a buffer zone helps immensely as the bees and other insect will make stops along the way.
Many plants of course do need insect pollination. The amount of distance needed for purity varies a lot between genuses. Having a nice sized grouping of the same plant and no others of that type in your garden will help a lot as the insects will visit many plants in the grouping which of course lowers cross pollination.
Some plants like say sunflowers have a long distance of at least 1/2 mile to keep pure, and if you have neighbor growing them way down the road, you of course can have some crossing. So for those types hand pollinating and bagging would be the way to go. If is isn't a rare type, it might not be worth the trouble. That of course is a decision to be made about the particular variety.
Not as many, but some plants are wind pollinated and the distance for those to be separated can be miles. Bagging is completely necessary.
Then there are those plants which must be pollinated from another plant of the same species or no seeds form. This doesn't happen as often with flowers, but there are some that will not develop seeds without another plant nearby. Apples are a well known example of this.
Hope this helps some,
Remy