When I finished the mission on Rannoch, Tali took off her mask, but I didn't get to see. Is there any way I can see her face?
Yes there is. If you speak to her on the Normandy after that mission, she will tell you that she left something for you in your cabin.
If you then go to your cabin, you will find a picture of her without her mask next to your bed.
This only occurs if you've romanced her though.
If you romance Tali, she will send you a photograph of herself without her mask on to your cabin. See the wiki for more.
First off, you need to have romanced her in ME2 as Male Shepard. She will not give you a second chance to romance her in ME3 like with Liara.
Finish Rannoch (last mission is the Reaper Base) and talk to her in the Normandy. She'll say she left something in your cabin. Head upstairs and on the left side of your bed is her picture.