Yes, iPhone 3Gs is faster and it have built in Video App. But physically its hard to distinguish with iPhone 3G. Both is almost same externally. Here we have got a good image guide from UnlockAlliPhone.com to find it out.
This image shows a very easily notiable difference. Here are some more difference we gathered.
# iPhone 3Gs is a bit heavier then iPhone 3G, but its heard to distingguish humanly.
# if you got iPhone in working condition, just open camera App, In 3Gs there will be flip option to switch between video recodring and photo shoot option.
# iPhone 3Gs is a lot faster then 3G. Just try to open iPod or any lengthy games. iPhone 3Gs will startup pretty quick then iPhone 3G.
# Another best way is directly via Apple site. Visit this link
Put your iPhone serial, you will be shown which iPhone it is.
# Visit this page for Apple Official details
You could always get support from UnlockAlliPhone.com about any Unlock & JailBreak issue of any model of iPhone including iPhone 2G,3G & 3GS and also upcoming iPhone 4. They provide Premium Members Support for life time. Means just pay one time a small fee and you will receive support(real human support – avg response time 2 hour) for all present & future version of iPhone.