You can check your TMS value by accessing the War Terminal within the War Room on the Normandy, so before you start Act 3 and pass the point of no return you should ensure you've reached the level you require for your choice from the Mass Effect 3 endings. At a TMS of 1750 or above you can choose between those two options, and at 2800 or more the third Synthesis ending is also available. The Total Military Strength you've managed to amass will directly affect the Mass Effect 3 endings you have available, as with a TMS of less than 1750 you will only have one possible outcome – either the Destruction ending if Shepard destroyed the Reaper Base at the end of Mass Effect 2, or the Control ending ending if Shepard saved the Reaper Base.