You will never before. You can use perf planner to get an idea of volume and CPC cost for keywords you want to target. But google ads is just one piece of the puzzle. Does the offer of the website is attractive? How is the website design ? How easy the lead form to be completed ? You can have the best ads campaign, of the other elements are crap then your cost per lead will be very bad. So you need to run it first to have an idea of how it performs based on your own situation.