Checking before fermenting:
If you want to measure the OG (gravity before pitching the yeast) take a sample of the wort in the fermenter before you pitch your yeast (or from the kettle if you're doing full sized batches in a large kettle and not diluting later) and let it cool to 60°F in your graduated cylinder and insert your hydrometre.
Remember to well clean and sterilise ANYTHING that comes in contact with your wort in the fermenter (wine thief, laddle etc)...
After you take the reading and THROW OUT THE SAMPLE (or drink it and take notes on the flavour to compare against tasting notes later).. do NOT add it back to your fermenter or kettle..
I sterilise my laddle and take a sample from the kettle 5 minutes before I start the wort chiller (to avoid contamination issues) and let it cool by putting the wort sample in a graduated cylinder and placing the cylinder in a cold water bath until it reaches 60°F.
Only insert the hydrometre into cooled liquids...
Checking after fermenting is done (before bottling):
Before bottling and adding the corn sugar, take a sample (with sterilised equipment) the same as above..
Side note: if you're looking for detailed info to learn when and how to do things like this, any good beginners book will explain how.
Cheers,
Awfers