If it's a one off domestic then I am not as professional as the above posters. I tend to lick my finger and stick it in the air to see which way the wind is blowing.
So let's say a custy says my Mum would like her windows cleaned, any chance of doing them? I find out whether the windows have been done recently (not usually, surprise surprise) and so if it's say a £15 house on a regular basis then I will say abot £30 cuz it's not done often and a bit out of my way (Say up to ten miles) and I will try to fit it in earlier or late in the day - say 8am or 5.30 if the light will let me.
If it's regular then I usually build it into the price.