Welcome to TutorLoL
Use the item Comparison tool to find out what item would be better in terms of damage. Be aware that it may not be necessarily the best option in terms of efficiency. To use it, select two items that you intend to build in your current game and read the tables to take your decision.

Remember that it is possible to open the page in your cellphone or ask for a friend to assist you by sending him you game code to assign your account. Use it when you're streaming to your friends on Discord.
By using the Combo tool, it is possible to see how much damage every ability would deal combined, place it in the correct order because it counts current health percentages. E.g if Blade of the Ruined King is selected after all other skills, its damage is going to be lower than if it was placed in the beggining.

It will not consider some damage amplifications, because of that some times it will display less damage than it would be in the reality, but never lower.
Use the ability section to see how much damage each ability deal, rune or item to any target with precision. Specially with mages, when analysing these datas, it is possible to determine if you are mostly or less likely to achieve a kill in a certain target.

Also, look at the stats section when playing ADC to see predictions of how much time it would take to kill a target based on abilities and basic attacks. Use it as a way to discover new builds.