There are any number of reasons why tyres develop leaks. These range from wear issues and leaking valves, right through to the unlucky and un welcome puncture.
First up, inspect the tyre you suspect has the leak, looking for obvious signs of wear or items like nails that may have been picked up on the road and become embedded in the rubber.
To check for a leaking valve, simply remove the cap and put a little saliva on the valve. If it bubbles, you have a leak. If there are no bubbles, the valve should be fine.
If the valve is fine, this may mean you have a small leak or a slow leak from a non obvious puncture.
You could test this at home yourself by removing the wheel and putting it in water to check for bubbles, which can be time consuming and difficult to do, or better advice is to take it to a trained professional at your local Repco Authorised Service Centre and ask them to test it for you.