I think this will work...
Code:
<<<SNIP>>>
        EEAddress=0
        I2CREAD I2cData,I2cClock,$A0,EEAddress,[CheckPanelDigit]
        pause 50
        gosub TestHex
        if ValidDigits = 1 then
            EEAddress=1
                I2CREAD DPIN,CPIN,$A0,EEAddress,[CheckPanelDigit]
                pause 50
                gosub TestHex
                    if ValidDigits = 1 then
                        Goto Vista
                    else
                        goto SnifferErr
                    endif
        else
            goto SnifferErr
        endif
        goto MainLoop

SnifferErr:
    LCDout $fe, 1
    LCDout $fe, 2,   "       ERROR        "
    Lcdout $fe, $C0, "PANEL NOT RECOGNIZED"
    Lcdout $fe, $94, "CHECK CONNECTIONS OR"
    Lcdout $fe, $D4, "SELECT CORRECT PANEL"
GOTO MainLoop

TestHex:
    ValidDigits = 0
    TestDigit = CheckPanelDigit & $0F           ' Isolate the lower nibble
        if TestDigit < 10 then                  ' First digit is 0-9
            TestDigit = CheckPanelDigit >> 4	' move high nibble to low nibble
            if TestDigit < 10 then              ' Second digit is 0-9
                ValidDigits = 1                 ' Set flag for good digits
            endif
        endif
return
How does that look? Am I thinking along the correct lines?

Thanks guys,
Chris