Like ?RecentChanges. The advantage is that it is less likely to get confused with normal text (at least in English). It looks less ugly if the link is at the end of a sentence which is also a question, because you don't get two question marks in a row.
If the page is rendered in plain ASCII, instead of HTML, the difference can matter. I once used a question mark prefix as a LinkPattern for one-word links, and it seemed natural to use the same syntax to render them. I couldn't have done it with question mark-after.
Why show links to non-existent pages any differently to normal links? Show RecentChanges as RecentChanges even if the page doesn't exist yet. This avoids breaking up the flow of text with weird metacharacters.
It makes it harder for readers to know whether clicking on a link will lead to useful content. However, that isn't necessarily a bad thing. If they are interested enough to go look at the page, maybe we can persuade them to fill in some useful content while they are there.
This is used by EverythingAtSlashdot?. Links to empty pages look the same as other links; if you follow one then it asks if you want to provide a writeup. It works well at Everything because people who use Everything are at least as interested in writing as they are in reading. (The basic act of participating in Everything is creating a new node.) Many WikiWikiWeb sites on the other hand are sometimes used as references by people who just want to read what's already there. This syntax would (for better or worse) disrupt such passive reading.
Pre-links could be displayed in a different colour, or with no underline, or similar subtle differences.
This is probably a bad idea for any WikiWikiWeb that is tied to WorldWideWeb technology, because it runs into trouble with browser support, and accessibility to the ColourBlind, those using text browsers, handheld computers, etc. It may have its use in some WikiWiki-like things, however.
Another way might be to change the straight underline to a wavy underline for non-existant pages. But you can't do that on the web either (yet).
But if this is done with a CascadingStyleSheets class, the user is free to set their own colour.
Placing the question mark after isn't bad because it's similar to the comment practice of putting (sp?) on misspelt words. e.g. "mispelled (sp?)". It naturally flows with English readers' expectations of question marks appearing at the end of questions. Perhaps like Spanish, you could use the upside-down question mark prefix but that is foreign and weird to anglophones.
The display of wiki links (both existing and non-existant pages) seems like a good area for user preferences. Some people may prefer color differences, while others may prefer prefix/suffix characters. (An "*" character in a smaller font might be a popular choice.) For both links, another user option could be to display spaces between each word.
The possible downsides are that rendering preferences would slightly increase processing time for page, and that customized versions are not (effectively) cachable on the server. (The server cache could store the default rendering--most people do not change default options.)
Of course, this idea would require user options, which are not yet implemented in most wikis. User options are planned for UseModWiki 0.9, which will probably be released several weeks after version 0.8.
To go a step further, look at the size of the target document. Show a red link for a empty or non-existent document and blue for a document at least 8kB in size. Use several other shades of blue/purple/red for sizes in between. This provides the additional information to a reader regarding the size of the target page. But a better indicator of "document quality" would be the number of revisions. Zero revisions (document doesn't exist) could be red and five or more revisions could be blue. From 1 to 4 would be shades of purple. Using red and blue is one of the WikiPedia styles but that site doesn't support gradients. Instead of link color, you could also use link size. But varying text sizes would make the text look funny. Again, preferences could fix this.
It is also possible to use an image insteed of a question mark. But for accessibility, this image need a good "alt" tag. One could also use the "title" tag to give informations about the link : does the page exist or not ? is the page on this wiki or elsewhere on the net ? what is the size, the popularity of the page ? who is the creator of the page ? date of last modification ? will the page open in a popup or in the same window ? ...