All iPad 3 models have a retina display. I own an iPad 3 and just recently updated it to iOS 8 without any problems. If your iPad does not have a retina display it's either an iPad 2 or the first iPad Mini. The iPad 1 can not be upgraded to iOS 8.
mrblue, I have 3 questions, why do you want to update to iOS 8? Do you have a current backup of your iPad and do you realise that you will need to update to OS 10.10 (Yosemite) on your computer in order to use the Handoff features of iOS 8. Not only that but because Handoff uses Bluetooth 4.0 (LE) technology only Mac Book Pro's made after mid 2012 and iMac's late 2012 will be compatible.
This is a good article on the subject; Yosemite's 'Handoff' will work only on Macs less than 2-3 years old | Computerworld
Having said all that iOS 8 is jam packed with changes that will no doubt have you fiddling with preferences and settings for weeks.
Last but not least if you have not received notification of an available software update it may be an indication that you device is not compatible. The above article tells you how to determine this.