mobile

Here's my situation. Last year I bought a Samsung Galaxy S4 unlocked AT&T (SGH-i337) phone from ebay. I am using Cricket. A few months back I tried updating it by going to the update area and checking for an update. I am on 4.2.2 and wanted to upgrade to KitKat. Well my phone tells me every single time it's up to date even though I know it's not. I decided to use Kies to see if I could update it using that instead. Kies tells me my firmware is also up to date. I gave up for a few months.
Now that Lollipop is out for my phone I tried once again doing the same things. Once again both ways say my phone is up to date. I looked through multiple forums to finally find a possible answer. My phone may have been rooted at some point and apparently if it was, it can't be upgraded. I read it needs flashed back to stock. I looked at a nice long guide on here on how to flash back to stock and saw you need to know which boot loader you have?
Please help me guys. I'm assuming my phone was rooted and have never rooted myself but I want to flash to stock so that I can upgrade to Lollipop. Any help would be greatly appreciated!