There is [this handy query](http://data.stackexchange.com/ubuntu/query/104525/most-common-tags-with-missing-wiki) on data.SE, which returns tags with missing wikis.

The [new](https://askubuntu.com/tags?tab=new) tab on /tags is a good place to start - these are tags that were created recently and thus are very likely not to have a tag wiki yet.

Other than that, you can also just click through the pages on the [popular](https://askubuntu.com/tags?tab=popular) tab and see which tags lack wikis.