No Oil filler plug means its a GREASE bearing, if it has the clear hub or the Plug its an Oil filled bearing.....they do get discolored but thats the older ones, the new Stemco hubs wont discolor, but you must stop the vehicle and wait a few seconds to check it.....I grab my wheels at the top and pull back and forth, is there a large amnount of give and take, if so your wheel bearings may need replaced or retorqued, and yes you should torque these....if theres any sign of Oil, Dirt collecting on the Hub yes you have a leak....dont over tighten these its cast Aluminum or Plastic....If you smell something HOT, or burning you should stop at once, seals break, lube leaks out, but you should have some warning.....simply placing your hand on the wheel near the hub and seeing if its HOTTER THEN #$%$# or is it the same temp each time, warm...are all the Hubs the same temp....Brakes smoking, surging, grabbing can be a sign of Wheel seal issue and lube on the BRAKES.....any good PM should find this easily....if they really do it !!!!