Yes, you can use the IR with all the components in place, eng temp normal, not running. Since you have the 2 piece shroud, its no trouble to get it out of the way. Measure the temp of the upper and lower hoses to give you a base reading and go from there.
I started doing this because I got tired of burning my hands diagnosing these things.
As for the clutch lock issue the specs Joe mentions are correct. I can say that the clutches on both of our 124s come on before the 107C. They usually kick in at approx 100C, sometimes less. I hear them roar alot during the summer months. The most common cause of fan clutch misdiagnosis is the center part of the radiator has become plugged not allowing air to flow through and heat the bi-metal in the fan clutch. I would like to add that my 89 300E has 156k on the odo, original fan clutch, same for the wife's 93 300E with 112k miles.
92 Volvo 740 Wagon aka "Mutt mover"