If you understand 12V DC electrics and have a VM or DVM it's real easy to check and diagnose the charging system.

If not, then here's an easy (rudimentary) charging system check...

Start tractor and let it idle.
Turn on the headlights.
Run tractor up to WOT while watching the headlights.
If the headlights got brighter then it is charging.

Since there are several different charging systems on a Briggs, you might give us a few more details so we can better tell you how to trouble shoot things.

Your headlights can get a lot brighter, but if you have the separate lighting coil type alternator, that won't tell you much about battery charging.

What tractor is this on? Give a model#. For example, if it's a Craftsman, give the Sears 917.xxxxxx number.
AND/OR the Briggs MODEL & TYPE numbers.

BTW, saying 18 HP Briggs is about as descriptive as saying Chevy. There's oodles of 18 HP Briggs out there from oodles of different models.

Bill is right on all counts.

Walt Conner