How to check the oil

Park the car on a level spot, turn the engine off and set the hand brake. Locate the engine oil dipstick, pull it out, wipe it off with clean cloth and insert it back. Pull it out again and have a close look. If you've never checked the oil level before, follow this link for detailed guide How to check engine oil.
If you find the oil is completely black (although for the Diesel engine black oil is normal) and (or) the oil level is very low (left picture) - suspect excessive oil consumption and (or) lack of maintenance. In either case the engine will more likely to have problems in the future. Another sign of poor maintenance would be dark stains (carbon deposits) covering the oil dipstick along its length.
Well maintained engine will more likely to have cleaner oil and the proper oil level (right picture), although this doesn't necessarily mean that the engine is in good shape; the oil just could have been changed recently.