Posterior to the cloaca.Here is a link to a photo that member chriswizz posted a few years ago with circles clearly showing the proper area that we need to see for sexing. Notice that these areas are anterior to and on the peripheries of the cloaca. What we are looking at are those really small scales within those circles, that indicates a male (as Roadkill has had to explain so many times before). The tegu in the photo has his legs spread out to show those areas clearly, which means pulling them slightly toward the abdomen and a bit toward the back, so there is no skin bunching up. We don't need to see the tail or even 2 cm anterior of the cloaca so get in nice and close. We also don't need a straight on shot to see both sides of the cloaca, one side would do and you can get better focused in that way. Basically a straight on shot of one of the circled areas with no skin bunching up and we should be able to sex it. Remember that these small scales are hard to make out if the photo is too bright, you need some shadow to see them clearly.
http://s1208.photobucket.com/user/chrisswizz/media/my tegus 2012 onward/IMG_1894-Copy2.jpg.html