How do you tell if a tigress is pregnant?

Salamanders do not get pregnant. They reproduce by spawning; that is, the female lays the eggs and then the male fertilizes them.
Salamanders don't get 'pregnant' as such. They lay eggs and when they are ready to do so they become 'gravid'. Being 'gravid' means that the female is ready to lay eggs and there are eggs within her body which are ready to be fertilised. Female salamanders will swell up when they are gravid and will look very fat. When introduced to a male they will mate and the female will lay her eggs and return to normal size.