Nobody knows. They need to keep their suits on outside their ships because they have almost no immunity system due to them keeping the floatilla so clean. If Tali were to take even her helmet off during a mission she would most likely get sick and even the smallest germ would eventually kill her. However, i belive in the Mass Effect Concept art book you can see that she has a normal nose and two eyes. Also, if you look at her feet, she has 3 toes (mayby a fourth one in the back).
Judging by the eyes behind the mask and the structure of their feet, it is possible that they have a reptilian appearence. However, nobody can know for sure.