First, you need to determine the exact model of the Ruger, as they make several types. It should say on the barrel, for example Security Six, Blackhawk, Redhawk, etc. Then you should note if it is blued or stainless steel (shiny finish) and measure the length of the barrel. If the grips are factory installed, the Ruger emblem should be imbedded in them, otherwise they might be custom or replacements. Finally, note the condition of the gun. Are there any dings or wear that show signs of use or abuse?
Now, you are ready to determine a price. The first place I would check would be
www.gunbroker.com and search for the specific model you have. See what others are asking for the same or similar gun. Remember, the value is not what someone wants for his/hers, it is what another is willing to pay for it. You can also go to your local gun store and see what similar guns sell for new. A used gun, even in great shape may not sell for the same price as a new one. Be cautious of someone who is anxious to buy it if you ask for a value. There are many greedy and dishonest folks out there. Tell them it is not for sale, that you would like a value for insurance purposes.
Good luck,
Tom