If you have a tank and bowl lift the cover and flush while looking in the tank
See if the tank bulb/flapper drops before the proper amount of water drains out.
Check the water level mark Or make sure the water level is about 1″ below the over flow tube.
If you have a flushometers depending on the brand where the volume adjustment is.
Sloan Royal has a flow ring and Coyne Delany has an adjusting screw just like the Sloan Gem UNLESS it is a 1.6 GPF
You could also try dumping a 5 gallon pail of water into this bowl to make sure you do not have any restrictions If you do then you will need a closet Auger to snake out this toilet.
If you have trouble with a flushometers type check your supply pressure as anything less then 25 PSI will not allow this device to work properly