Tegus are sexually mature around 3 years old. They are sexually dimorphic, which means that males and females look different from one another. Males tend to be larger than females, have a much larger head, and display prominent jowls underneath the chin. However, jowls are not always a reliable indicator of gender.
In the case of Argentine tegus, they can also be sexed at around 36” long by feeling the area around the vent with your fingers. If there are bumps at the sides of the vent (commonly referred to as “beads” or “buttons,” comparable in size to BBs), you have a male. Attempting to sex earlier will have inconsistent results.
At the end of the day, the most reliable method of sexing a tegu is having him/her probed by a certified reptile veterinarian. But this video here is pretty helpful: