The first step is to search the wiki thoroughly for existing tags that describes the same concept.
The second step is to [search for undocumented tags][3] that describe the same concept. It may not always be possible to deduce what those undocumented tags are being used for, especially if the none of the tagged objects are physically near you. So this step is really optional.
In this case the new tag seems to be a trivial extension of an existing scheme. But, if you are still unsure, you can ask your question on the [tagging mailing list][1].
After that you can [start a new page][2] for your tag. Doing so will prevent many (semantic) problems like honomyns or honomyns, tags based on words that are not specific enough.
enough and acronym namespace conflicts.
A wiki is a remarkably powerful tool for collaboration. If experienced OSM users see a problem with your tag (e.g. being a duplicate of an established tag), they can [easily see how many times it has been used][3] and take appropriate action. For example, they can add a message to your wiki page.
[1]: http://lists.openstreetmap.org/listinfo/tagging
[2]: http://wiki.openstreetmap.org/wiki/Tag:surface%3Dduckboard
[3]: http://help.openstreetmap.org/questions/1021/where-can-i-see-which-tags-are-in-active-use
