Where is the oil pressure sensor located on my truck and what does it look like? How do you test it?

My oil gauge goes from 60lbs/pressure down to 20lbs/pressure and all over the place when it warms up, it just started doing this in the last week. I thought maybe it was an oil pressure sensor that needs replacing, well I got out my Chilton's book and found on my truck what I thought was the right sensor until I bought one now I'm not so sure. I thought it was under the cap where you add oil theres a hot wire coming out of it going to a connector which has a ground on the other side of it but the sensor that they gave me doesn't look like that so I'm asking for some kind of mini diagram to show me exactly where it's at and if I'm on the right track. Thanks