I would first check the moisture barrier bag that the parts come in. There is a label showing the MSL level on the bag and sometimes included with the information on the reel. Transfer this info to your MRP system. If your receiving department throws away the bags they come in, you have more work to do.
If worse comes to worse and you just can't get the information, my rule of thumb is to assume that all IC's are at least MSL3 and treat them that way. I wouldn't worry too much about the R's & C's. I've found that some LEDs and large IC's can be MSL4. Assume bare PCBs are MSL4.
Tin/lead is more forgiving than if your process is lead free. Some readers may disagree with some of my points above but it's just my opinion. Hope it helps.