One way to test for a leak is to do the bucket test. Fill a bucket to a certain point and set it out on the pool deck. Both the pool and the bucket should lose the same amount of water from evaporation.
As TaterGun said, the bucket test is the simplest way to convince yourself. Personally, I like to put the bucket on the step inside the pool so that it stays at the same temperature as the rest of the pool water. Put a heavy rock in the bucket (so it's not going anywhere), and equalize the level of the water in/out of the bucket and let it sit on the step inside the pool or 4-5 days. If you can detect any difference in the level inside vs outside the bucket, then you have a leak.
Make sure it has enough height above the water line to keep the waves and splashes out. I like to pull it out of the water when the kids are playing, just to remove the temptation of them using it as a toy.
I used to do that and found I was losing a lot of water. Couldn't find the leak so I hired some pros. They spent 4 hours and couldn't find the leak either. They put some water in the pool and then pulled the hose out. Cost me $325 and I still didn't know where the leak was.
Guess what? Yep. Siphoning out through a leaky hose!
As many others have stated, how often you add water depends on the temperature, amount of sunlight, water loss due to splash-out, and whether there are any leaks, to name the most common factors. What I would suggest is that you need a baseline to help determine what is "normal" for your pool. Beginning with your next top-off, I would keep a log of all top-offs. After a year, this will give you a baseline that will account for seasonal variations. Once you have this baseline, any sudden variances will become readily apparent.
In Texas we just leave the hose hangin' over the side in August..............