Getting the Hardware - more
Your safest bet for making sure that a product has a mini-PCI board is to look up the FCC ID of the product you're thinking of using by using the FCC's Equipment Authorization System Generic Search page. Just enter the FCC ID number obtained from a label on the product (unfortunately not on the product's box, however) and click on the Detail link in the Display Exhibits column of the search results. You then want to download or view the Internal Photos file (you'll need an Acrobat viewer for the PDF file), which will quickly tell you whether the product has a mini-PCI card.
If you're thinking of attempting this hack, you should know what to look for. But in case you don't, the two pictures below show examples of products with on-board (Figure 2) and mini-PCI (Figure 3) radios.
Figure 2: On-board radio
Figure 3: mini-PCI radio
If you don't have the product in hand, try a Google search using the model number of the product you're considering using along with "FCC ID". A search for wrt54g +"fcc id" in both Google Web and Groups comes up with the FCC ID of the original version - PKW-WM54G - in the first page of search results in both cases