If your B12 levels were low, then this would be indicated on a regular full blood count - you'd have low haemoglobin levels and large red blood cells. If this stuff has come back normal, then it's very likely that your B12 levels are normal.

Regarding the effect of gastric bypass surgery on B12, absorption of B12 is dependent on a substance called intrinsic factor for absorption to occur. Intrinsic factor is made by the cells in the stomach and then B12 is absorbed further down the gut after binding to IF. So if B12 levels are low after gastric bypass surgery, it's not going to be fixed by oral B12 anyway because it's the intrinsic factor that's the problem, not the actual amount of B12 ingested.