Case law holds even convictions prior to 1994 are good for enhancement under PC 49.09(a) and (g). But a defense attorney says many 1980s cases are not eligible because of invalid jury waivers. If true, how to know which ones?

Are you talking about misdemeanor or felony cases?

Prior to 1991, TCCP article 1.13 only required felony cases to have written waivers. So, if it was a misdemeanor, written jury waiver was not required.

"The defendant in a criminal prosecution for any offense classified as a felony less than a capital shall have the right, upon entering a plea, to waive the right of trial by jury, conditioned, however, that such waiver must be made in person by the defendant in writing in open court with the consent and approval of the court, and the attorney representing the State."

Also, remember:
“Recitations in the records of the trial court, such as a formal judgment, are binding in the absence of direct proof of their falsity.” Breazeale v. State , 683 S.W.2d 446, 450 (Tex. Crim. App. 1985)(on reh’g). These records carry “a presumption of regularity and truthfulness.” Id. at 450-51. Therefore, if the judgment of the court recites that a defendant waived his right to a jury trial, “such presumption attains until and unless the contrary is made to appear.” Id. at 450.

I would think the practical answer is "The ones that said defense attorney proves to you are not eligible for enhancement."

Thank you Brody and Andrea--great advice.