Other than experience showing you, the easiest way is to add the newly created symbols to the design, both Schematic and PCB.

If a Schematic symbol, add it to the design and check its size against other items along side 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.

If a PCB footprint, again, check it against other similar items for size and look. To be honest, checking PCB Footprints isn't easy. Previous experience has shown that if you have a physical package sat in front of you on the table, you can do worse than printing the footprint out onto paper and placing the physical device over the print to see if it fits.

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 you haven’t messed up the Component definition. Adding Logic Names to the Component also helps this process.