folate will not affect the results of tests specifically looking at B12 and neither will it affect the result of tests for PA - one specific condition that is the most likely cause of a non-dietary B12 deficiency. It can reduce the impact of macrocytosis which is a potential symptom of B12 deficiency. However, this is only a symptom and not a defining characteristic of B12 deficiency - though most GPs are unaware of this.
At least 25% of people presenting with B12 deficiency will not show any signs of macrocytosis. Whilst some of this may be down to folate levels, some may be due to iron levels being low I have not seen any studies that show this is always the case and there are other factors involved - it is, afterall, a symptom and people vary as to which symptoms surface first. I have had neuropsychiatric and even some neurological symptoms for decades before I was identified as having low B12 but have never shown up with any noticeable anaemia - so my GP (mistakenly) believes that they caught the B12 problem early.
Folate deficiency will also cause macrocytosis - larger rounder red blood cells.
There is an extremely small risk that supplementing with large doses of folate in the presence of a B12 deficiency could cause SADC - a neurological problem. This has been observed in fruit bats and there are some isolated reports of this happening in humans. Folate supplementation should therefore start 24-48 hours after B12 treatment have started if both are deficient.
The two tests currently used to clarify a potential B12 deficiency - MMA and homocysteine are both looking at waste products that build up, causing other problems, if the body doesn't have enough B12 to recycle them. They can also be raised by other factors which is why they aren't front line tests. One of the things that will raise homocysteine is folate deficiency, so it isn't a good test to use if serum folate shows low levels of folate. MMA isn't affected by folate levels.