Can I mix soap and water to find leak... - how much soap / water??

Don't use soap, use dishwashing liquid. Mix at about 25% /75% water. Brush on the fittings. If this is a new home or permitted rehab. this may not be a code approved method. Some require specifically made liquids and others will require a pressure test.

Mr. Bubbles works great for this.

A little squirt of Dawn dish detergent in a cup of water. If you see bubbles, you have a leak.