Sometimes we can tell; but often we don’t know if your bow needs a rehair or not, only you can answer that.
We can tell if the hair is obviously very old and oxidised and we can also tell if a lot of the hair is missing or broken, but we can’t readily tell if all the hair is in place and it looks reasonably fresh.
So, how do you know? Well, the hair is covered with microscopic ‘barbs’ or scales which grip the rosin and, as you play, the friction against the strings slowly wears these barbs down and sometimes tears them off. Over a long period of time (conscientiously practising) so many of these barbs are damaged that the hair no longer grips the rosin and you find you have to keep putting in more effort for diminishing results.
Your bow is now losing the essential friction with the string to easily produce the sound you want. You try to apply more and more rosin but it just falls off onto the front of the instrument without improving the sound (at which point you suddenly remember why you have that clean dry duster …!). Now you know you need a rehair!
And how long between rehairs?
Again, we can’t say exactly because of different playing pressures and techniques and how much playing is actually being done on a daily basis, but for a professional or advanced student, perhaps from three to six months would be reasonable