Typically, the company that made the sealed unit will put a sticker INSIDE the sealed unit, either on the aluminum spacer or on an inside surface of the glass so that it can't be tampered with. The sticker will identify the company that made the sealed unit so they can check right away if it's one of theirs. And, they won't give you a new one unless you can prove that it's still under warranty. (Maybe the purchase agreement on your house if you bought it new would prove the approximate date of manufacture. And, if you're not the original owner, they're likely going to say that the warranty is for the original purchaser, and doesn't extend to anyone who subsequently buys the property.
Also, the builder may have said that the windows have a "lifetime warranty", but that is very ambiguous wording. The "lifetime warranty" probably applies to manufacturing defects. That is, over the life of the window, if the window is found to have been manufactured incorrectly, they'll either fix the problem or offer you a cash settlement if they can't fix it. Or, it could be a PRORATED warranty, just like tires have on them. That is, the warranty may be for 10 years on a tire, but each year the amount the manufacturer will pay diminishes to account for wear on the tire. If your tire has a blow out in the 9th year, the manufacturer might only pay out $20 per tire (if they originally cost $200 each).
NO window manufacturer is going to offer a lifetime warranty on it's sealed units. That's cuz no sealed unit is going to last forever. You can expect to have seals around the sealed unit to start leaking any time after about 10 to 15 years. Lots of windows will go 60 years without having the seals leak, but no one is ever going to guarantee them for that long. They typically guarantee them for 10 or 15 typically, but a lifetime guarantee on the sealed unit would be foolish on the part of the window company because eventually, all of those sealed units are going to leak and need to be replaced.
What I'd suggest you do is measure the window and phone around to the places in your area that make sealed units to get cost estimates to make up a new one. Your measurement doesn't need to be accurate as the place that quotes you the least for your measurements will also quote the least for the actual size of the sealed unit you bring down to them. (And, while you have each company on the phone, run the idea of a lifetime warranty on the "window", and see if they agree that warranty would also cover the sealed unit against leakage. They will unanimously agree it wouldn't, and would probably refer to manufacturing defects (which are typically found in the first day or two after installation)).
For a triple glazed dual argon filled sealed unit with two low-e coatings, I got quoted prices ranging from $13 to $29 per square foot here in Winnipeg about a month ago. That's quite a wide range. I ended up going with a company that makes sealed units for other window companies here in Winnipeg.
It's not at all hard to take the sealed units out of modern PVC window frames, and all you need is something called a bee keeper's bar. You can buy these in any hardware store, and they're typically called "lever scraper bars". However, if you look under "Apiary supplies" or "Beekeeper's Supplies" in your yellow pages, the places listed there will sell these bars in different sizes and styles, depending on the kind of honeycombs the beekeeper has. They're made for scraping the wax and honey off the honey comb.
If you can't find the window manufacturer's name anywhere, and are forced to replace the windows at your own expense, then you can use the above tool to remove PVC window stop moldings that are typically used to hold the sealed unit into the frame. If you take one of them out, look for a sticker on the outside of the sealed unit that might identify the manufacturer. Actually, it's best to take the leaking sealed unit down to the place that's going to make a new one so that they can be sure to make it to the same dimensions as the old one. The last thing you need now is a dispute over what dimensions you stated over the phone. If the company has the old sealed unit, they'll make it the same, and there won't be any dispute over what size it SHOULD have been.