Research the battle of the Alamo in an effort to get the facts. The twelve-day siege lasted from February 23rd until March 6th, 1836, pitting 200-odd Texas defenders against over 2,000 Mexican soldiers. Despite the staggering odds, the Texans repulsed multiple attacks before being overwhelmed. The fight served as a rallying cry for the rest of Texas, which eventually led to independence from Mexico. The Alamo's official homepage contains a number of details, as do multiple historical texts on the subject. The more you know about it, the better you can judge a given movie's accuracy.
Watch a specific Alamo movie with an eye on the events. Note how events are staged to appear more dramatic or rearranged to fit the convenience of the plot. Hollywood tends to take great liberties with historical events and some versions are more accurate than others. For example, 1987's made-for-TV movie "The Alamo: Thirteen Days to Glory" was praised for its attention to detail, while the 1960 John Wayne movie "The Alamo" is now widely derided for the liberties it took.
Study the characterizations in the film and compare them to each other. Beyond the historical circumstances, the famous figures who died at the Alamo have done much to secure its legend. In part, a given film's depiction of these characters must be accurate but an engaging onscreen personality carries weight too. John Wayne's Davy Crockett in "The Alamo," for instance, bears little resemblance to the real figure, but Wayne's onscreen presence is quite engaging. Conversely, many more accurate versions of the battle actually show Crockett in much less interesting terms. (Billy Bob Thornton's Crockett in 2004's "The Alamo" may have the best of both worlds: a charismatic and interesting personality plus historical accuracy.)
Compare the films in terms of the era in which they were made and the way that influences their approach. 1960's "The Alamo," for instance, portrays the Mexicans as uniformly villainous, while subsequent productions aimed for a more nuanced and subtle approach. (2004's "The Alamo" depicts Jim Bowie as a drinker and a slave owner and Davy Crockett as a borderline coward.)