How can one distinguish erythema migrans (EM) from a simple non-specific bug bite reaction?

The following suggest EM:

History: running about brushy, wooded, grassy habitat during late May through early fall (in midwestern and eastern United States) or January through May (in Pacific Northwest) w/ EM type rash developing 3 - 32 days later; history of tick attachment > 18 hours

Physical Exam: expanding annular lesion which may have several rings of varying shades of red, center may be indurated, vesicular or necrotic, maximum median diameter is 15 cm (rather large); can be associated with burning sensation, itching, or pain.

Diagnosis: Classic EM does not need serologic confirmation. In equivocal cases, serologic testing may be considered. IgM antibodies appear within 1-2 weeks and IgG antibodies within 2-6 weeks following onset of EM and can be detected using a sensitive (although not specific) ELISA assay and confirmed with Western blotting.

Of note, antibodies often remain elevated despite therapy and resolution of symptoms.

Source

Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology, 5th ed.

Sexton, Daniel MD. "Diagnosis of Lyme disease" Up to Date. <http://www.uptodate.com>