Is time dilation and length contraction hypothetical or is it really an observable fact.

for example,consider a rectangular sheet in a spacecraft moving at some high velocity.

Would an observer in the spacecraft be ever able to see the sheet as a square due to length contraction along its breadth(if the velocity is so so adjusted).
Would a stationary observer in the earth be able to observe the same.
In other words who experiences length contraction?

Also who experiences time dilation?

Asked Ajmal