You can check your engine oil when the engine is either warm or cold as it doesn't really matter.
If your engine has been running though and you shut the engine off you should wait at least 10 to 15 minutes after shutting the engine off to check your engine oil level.
The 10 or 15 minutes waiting time will allow the engine oil to return to the oil pan so that you get a more accurate oil level check.
You can also check the engine oil with the engine cold such as during the morning or before you get into the vehicle and start it up.
I check my engine oil everyday when possible just to be sure there hasn't been a leak which would be bad if all my oil leaked out and I started the engine.
So you can check your engine oil both warm or cold and it won't matter as you'll get the same accurate results either way.
But also make sure your vehicle is level and not on a slant so that you get an accurate engine oil level reading.