Primary symptoms of low stomach acid are acid reflux and gaseous bloating, especially after eating.
Best way to increase stomach acid are 50mg zinc daily with meals. B vitamins from food or supplement. Trimethylglycine (TMG) 3 times daily with meals.
If you are low in stomach acid you will have trouble absorbing the nutrients needed for stomach acid formation. So you may want to take raw apple cider vinegar with your meals for a while to aid in the absorption of these nutrients until your stomach can produce enough of its own acid.
Otherwise I would go with a sublingual B6, B12 and folate supplement, which does not require stomach acid for absorption. These are absorbed directly in to the bloodstream from under the tongue.