I use a dial indicator, a ball end tip. Place the phone glass down on a smooth surface that is slightly smaller that the outer perimeter of the glass. That ensures that the lip of the frame does not provide false data. Run the phone slowly along the work surface on the block your using so the tip of the indicator rides the back of the frame. You can use clear screen tape that is properly adhered to prevent scratches if needed. Now if the frame is bent, the tip of the indicator will rise and fall with the bend. That frame should only have a few thousands of a inch of travel for the indicator tip if it's straight. Any thing more and you have located the trouble area. You can use a straight edge or such to further check those spots. Once you confirm a bend you can use a icorner / gtool to straighten the side wall. Make sure the kit you buy does the side walls as well as the corners too. As far as links for the tool go, I'd say shop around. ifixit has one and ebay has many listings as well. Yes this process can be time consuming but (though if you practice a few times it becomes easy and takes a few minutes to do), I look at it like this. I did not bend it. I did not decide to use Reynolds wrap aluminum foil for a case on a 600 +dollar phone. Lastly I do not want a customer returning and need to redo the repair for free under warranty. I tell my customers to do it right or the repair is not going to work long term and they will be back. Never had a customer that did not understand and appreciate my honesty, especially when it potentially saves them from paying for parts and or labor for a second repair.