I noticed when I bought it that the slide release was aftermarket & there was a grip plug in it.  I liked the trigger but not sure if it was modified or stock, anything I should look for?  The gun appears to be rarely if ever fired.

Sights are stock, but that won't last long.  I haven't had a chance to actually fire the weapon yet.

Thanks for the help.

Take it and shoot it some to get a feel for things before you change a bunch of stuff.  That will tell you more about what you have to work with!

Thanks, only going to change to 3 dot adjustable tritium sights for now.  Will probably upgrade recoil spring & guide rod when I get some full power loads. 

I am just wondering how to tell if the trigger is stock or aftermarket. 

The slide release upgrade is common, and depends on the intended use of the pistol.  Good for range use and competition, bad for carry.  It shouldn't affect function, at all other than ease of use (intended or unintended).

The grip plug is purely cosmetic, and doesn't affect function in the least.  If you like it, keep it.  If not, remove it.

The trigger is a tough one.  You'll have to compare it to others.  If you like it, and find it functions without any faults, it's fine, whether or not it was upgraded.  Otherwise, the OEM replacement parts are not expensive, and very simple to replace.  Many aftermarket parts are marked different than OEM, or will appear different.  Compare what you have with pics of the OEM parts.  You'll have to detail strip the frame to see all of the parts.  Not difficult, but further than a field strip.  If you've never done it, there are several good videos online to help.  It's also helpful if you snap pics at each step as you go.

As a Glock Owner, look for a GSSF shooting event nearby, and visit the armorer that will be present.  I would bet he will put you gun back to "stock glock" for no charge.