Other than experience showing you, the easiest way is to create a component to allow the newly created symbol to be added to a design of the same type as the symbol

If a Schematic symbol, add it to the design and check its proportions against other items alongside it. Does it look too big/too small, are the pins too far apart/too close? Are the pins on the same grid or multiples of the working grid? If they aren’t, you’ll get lots of ugly short segments when connecting to them.  As a general note, schematic symbols in the supplied libraries almost universally use a 0.1" imperial grid, though there are one or two connections on a 50 thou grid.

If a PCB symbol, again, check it against other similar items for size and look. To be honest, checking PCB symbols isn't easy. Experience has shown that it's often simplest to print out a 1:1 paper copy and check the fit with a physical device.

Another important check for both Schematic and PCB footprints is to check the pin numbers when used in the design. Check that they match the manufacturers datasheet to ensure it matches the Component definition. Adding Logic Names to the Component also helps this process.

If errors are discovered, edit the PCB symbol in the library then use the Renumber Pins tool on the Edit menu, remembering to update any affected designs after saving the changes.