Dead easy if you have the label, but what if that is lost, or you have an established Clematis in a garden you have acquired?
Here are some hints.
There is an old gardening saying "If it flowers before June, don't prune." This applies well to Clematis. Those which flower early in the year, from February until early May are going to be Clematis alpina, macropetala and montana, which for pruning purposes, are all 'Group 1' The flowers are on growth made the previous year which means this type of Clematis must not be pruned early in the year in the spring, or you will cut off the buds and branches on which the flowers are forming, and cut off the potential flowers. This group is an exception within the Clematis group, as it does not require routine annual pruning and will flower happily without pruning. Group 1 Clematis can be pruned if needed, but always after flowering, to tidy up or contain within a growing space.
The next group of Clematis are very popular because they have large showy blooms and can be identified because they flower in early summer May and June, and are varieties such as 'Nelly Moser', (Image above center) 'The President Niobe' and are all known as "Group 2"
Finally, the third group are the late flowering Clematis. These are a bit confusing because some Clematis in this group have large flowers such as 'Jackmanii', (illustrated above) and others can have small flowers such as C. viticella. Although the flowers sizes are different, they are all distinguishable by the fact that all flower late in the season and are known as "Group 3"
If you make a note when in the year your Clematis flowers you can make a judgement on how to prune it.
Early spring flowering are group 1; summer flowering group 2 and late summer flowering group 3. This is a broad brush approach as the only fail-safe way is to identify the precise variety of Clematis, and then to check to which pruning group to which it belongs. Since there are hundreds of types of clematis and new ones being bred all the time, identification this way is not always easy.
Now you know which Clematis or pruning group it is - what to do next ?