There are few different indications that can tell you if you might have an obstructed fill valve in your toilet. The fill valve allows water to flow through it in order to fill up the reservoir tank with water. There are several issues you can watch for which might indicate that your fill valve has an obstruction. For these issues, you may be able to clear the obstruction, or need to replace the fill valve.
Step 1 - Running Toilet
Determine if you have a toilet that is constantly running, an obstructed fill valve could be the cause. If the fill valve is blocked so that it cannot close properly, it will allow water to continuously run into the toilet tank. You can try to remove the obstruction by taking off the top cover of the valve and flushing water through it. If this does not clear the blockage, you will need to replace the valve.
Step 2 - Slow Filling
Determine if your toilet takes an extended amount of time to fill the tank with water, there could be an obstruction in the fill valve. The obstruction is slowing the flow of water and extending the fill time of the tank.