Jeff is giving you some good advice, and if you have discolored exhaust, etc., he gave you a good place to start.
But if I read his post correctly, he neglected to tell you how to check the oil
You are right about the two petcocks on the side. Open the lower one. Oil should come out. If it doesn't, close the bottom one, open the top one, and add oil until it dribbles out the top one.
And now you know how to check the oil on a whole bunch of different kinds of antique machines, including good ol' Model T!