Check the format. The message must begin with a "-----BEGIN PGP SIGNED MESSAGE----- line and end with a "-----END PGP SIGNATURE-----", there must not be any blank character at the start of a line, all contents lines must appear as a square save the 2 lasts.
Examples (you won't be able to decrypt any, but your software must recognize the first one as a legit message):
Good:
-----BEGIN PGP MESSAGE-----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=turf
-----END PGP MESSAGE-----
Bad (missing line-feed: some lines aren't properly terminated)
-----BEGIN PGP MESSAGE-----
hQIMAzWDhPiExd3rARAAnqIe3Zz0hsk+Ce599bFYiOjEzQ55EZHsUAbU/J0iC1Tc D2ccGfCopfLHz/w+iT5SZbcMTVAL+LMP15KLAT7zzYJ3FnkZ3fBOT5SN4YMKcww7 uWI7N42p4XyDXVuiMkyYafnI6RstMvzQxf2O6oKEaxb7Ye3hY5g9tdo+Cz0M8ycr 2zbsBzL145WuPiE5Dpmh08fFs7pL+l+w+yqGBO5vk1r/Rabg0gZl8SKqtXtW2XGl s9s2t9Y8QSZBTrzXw3kHyEDLWeL1jS8RFhLAxscNFFNAXkA+3vUv71kDGk8IhD73 r8sKtyWQEN9lglea5ntV1VvtmvAgo/4QNORWilJBOl+66mIdvhaVP/13/ctkfua7 TuyqDoka1wLHstFNek8VX+AnG8GRY12jVELPqqKEB3KTN1DhsixOwtBd3ZuTzw6S AH2zWt9Q+okUNq+hipOLMYoyRYmUSbgeu8sQEzlVKsQLPTY364xtQTr5CKXZjY3T 7Wcw4NBmaht4QE+XDMk0kanlwWhGcV29+GTxwGEmPXvJIy6k9LgmcS8pJ5j/+I+w 5/FC5P4m/zC5tg98T/7cwweSszLGrQ6ktBDIpBTacQKl1Ezp7NB1Q5+foB9NVet8 WizdQr+G+jR4sKMTqiXuTRO52GaT0WEqQasDrO6oVf3sev4yd515WYhvcxaRru/S wA8B4vx7qVVjEA1sGwXMqooz05eV4G6/fJeVLES4UmI1PxYWeu6kWeSghtLxufUI kicN9dH16Cny/nfmJJMeWHpWR5Xy5a7KcVyMDBdcDvOyuiLMvAEg0Uy++15xD2JU qKBgr3I0240znWWQF+l2BKM+DSStEoy3DHpk7K9+ppb8WJZjONmVBGsANE736drL LmwsJXjWCujSCxu1i8w2vMupLRXi8KbWoo6ChfWc8/4fGscW8yyMw9mOwH0gffBg IrPzka/cYtgp4IFF8mgltyw= =turf
-----END PGP MESSAGE-----
Bad (missing dashes in the first line, too many dashes in the last one)
---BEGIN PGP MESSAGE-----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=turf
-----------END PGP MESSAGE-----