When the other end of an active teleporter is destroyed or picked up by its builder, the end I'm facing will stop glowing and spinning.
If the teleporter is destroyed, it will show up in the kill feed. However, that's fairly easy to miss, more so when my base is being destroyed by a rush (and this doesn't help as much as I had hoped). The end I'm facing will also reset to level 1 if it's upgraded, but if it was level 1 to begin with then that's not going to help much.
If an active teleporter is sapped, sappers will be planted on both ends, so that makes it pretty obvious. However, either end may still be destroyed prematurely leaving the other one intact and sapped.
If the teleporter is picked up, the end I'm facing will not reset to level 1 if it's upgraded. Again, if it's level 1, it's not a very helpful indicator. Note also that buildings cannot be picked up when sapped.
Are there any visual effects or sounds I can pay attention to, in order to tell quickly whether that other end was destroyed or picked up regardless of what level it is?