Ventral sexing is an estimate
Looking at the underside of a tarantula can suggest sex, but the method is less reliable than examining a good moult, especially in small or unfamiliar species.
What experienced keepers look for
Keepers compare the shape and features around the epigastric furrow and may look for epiandrous fusillae associated with males. Lighting, angle and species variation can make interpretation difficult.
Size matters
Very small slings are particularly hard to ventral-sex confidently. Apparent features can change as the spider grows.
Photos can mislead
A blurred phone photo through plastic is a weak basis for a confident sex call. Reflections, hair and posture can hide the structures being judged.
Use a moult when available
A well-preserved exuvia lets the keeper examine the internal structures directly and is usually the better route to a confident identification.
Be honest about uncertainty
If the evidence is ambiguous, label the spider unsexed rather than presenting a guess as certainty. Good records are more useful than false confidence.