By bumping the side you should be able to feel the calf by 6-7 months. Sometimes the calf will be laying in a position that you might miss it even at 8 months.
Bumping a calf is easy, you feel a hard spot floating around. But the cow has got to be standing still and relaxed. If your bumping a solid calf with out much floating. You got a calf at near or full term. What's her udder and back end look like?