Short answer: without a blood test, you cannot know with certainty. But you can estimate risk reasonably well, and the useful inputs are risk factors rather than symptoms.
Why symptoms underperform
Fatigue appears in the documented symptom lists for B12, iron, and vitamin C deficiency — and in dozens of unrelated conditions. Reasoning from a shared symptom to a specific nutrient is guesswork.
Vitamin D makes the point more sharply: NIH ODS notes the serum levels associated with deficiency "have not been definitively identified," and people are commonly below sufficiency thresholds with no symptoms whatsoever.
Why risk factors do better
Risk factors reflect mechanism — how a deficiency would actually develop in you — rather than a downstream signal shared by many conditions.
NIH ODS identifies these B12 risk groups: older adults (3–43% of community-dwelling older adults are deficient depending on definition), people with pernicious anemia, those with gastrointestinal disorders or prior GI surgery, vegetarians and vegans, infants of vegan mothers, and people on metformin or proton pump inhibitors.
For vitamin D, USPSTF names low dietary intake, little or no UVB exposure from winter season, high latitude or sun avoidance, and older age.
For iron, NIH ODS names young children, women of reproductive age, and pregnant women, with poor diet, malabsorptive disorders and blood loss as mechanisms.
The combination that actually matters
The strongest signal is a risk factor and a matching specific symptom. A vegan with tingling in the hands is a far more specific picture than either fact alone. That combination is what makes a clinical conversation productive.
What settles it
Only testing confirms deficiency — and as NIH ODS notes for B12, even a normal serum result may need MMA to fully resolve the question. Risk assessment tells you whether to pursue that; it does not substitute for it.
Our free assessment works through these documented risk factors systematically.
References
This article summarizes published guidance from the sources linked above. It is general information, not medical advice, and it cannot tell you whether you personally have a deficiency. Only a clinician ordering the right test can do that. If you have symptoms that concern you, speak to a doctor.