I would also like to add that even if the patient states they are voiding ok, check for full bladder. Some patients have a hard time emptying their bladders. I agree with the previous posters advice on not being too gentle for the first assessment. This will come with experience. A good way to get experience is to spend some time with the L&D nurses and observe/practice fundal massage with them.
When you do have a questionable PP hemorrhage.......make sure the patient is in the bed! Its better to have them make a mess in the bed, than pass out on the floor! On Christmas day a few years ago I answered the call light of a patient who was assessed by her nurse just 30 min before I came into the room. Blood everywhere-pph that responded to methergine, IVF with Pitocin, good fundal massage to express the clots. Patient turned out ok. Then after she was taken care of....another new nurse asked me to check her patient...because she thought she was bleeding a little too much. I walked into the room and observed the patient slumped in the bedside chair-blood pouring on the floor-pt very weak/lethargic. Called for help, did the usual routine and this patient ended up needing 3 units of blood. PPH can be very very scary. ALWAYS get help with these patients and you will do fine.
Its also helpful to go over "what-to-do" scenarios in staff meetings to keep your skills current for these emergency situations.