Anaphylactoid (or true anaphylaxis) reactions are exceedingly rare complications of envenomations, but do happen. Risk factors include previous envenomations. Anaphylaxis in first-time envenomations are reported only 3 or 4 times in the medical literature, but may be under-recognized as it's postulated that dermal or GI exposure (i.e. through chronic handling by long-term keepers) may cause the IgE-mediated allergic reaction that we call anaphylaxis. Complicating the picture is that severe envenomations can cause a distributive shock that resembles but is quite distinct from anaphylaxis.
You are much more likely to get anaphylaxis from anti-venom than from the bite itself. Even then, the medical community does not recommend prophylactic administration of epinephrine prior to anti-venom administration. this last point is an active topic of debate, however.
Epi-pens have their place for true anaphylaxis reactions, but probably do more harm than good in the overwhelming majority of envenomations. Signs of anaphlyaxis that mimic signs of envenomation include anxiety, nausea/vomiting, sweating, metallic taste, hypotension. Signs suggestive of anaphylaxis rather than snake bite include hives, generalized itchiness or flushing, generalized swelling (rather than localized to the bite), difficulty breathing (due to airway constriction as well as upper airway swelling). Death from anapylaxis is usually due to airway problems. Death from snakebites (crotalid) is usually due to an overwhelming consumptive coagulopathy and sometimes distributive shock, neither of which improves without neutralization of the venom (although intravenous epi would transiently help the shock). Epi-pen use in the absence of anaphylaxis could potentially exacerbate snake-bites by causing tachycardia (pushing the venom around) and worsening anxiety, as well as the other side effects of the drug.

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