Question on keyless entry system

I bought my blazer about 8 monhs ago, and after surfing ebay for awhile I decided it would be nice to have a keyless entry on it. As my blazer did not come with a remote I dont know if it originally did or not. Anyone know how to tell if it has the keyless entry feature? Also, how difficult is it, to program one that is bought from ebay or another source?

Thx