G.24.1- Some spells and abilities require a choice as to which mode they operate in. This choice is made on announcement (see Rule T.4.2). [CompRules 1999/04/23]
G.24.2- Cards worded as "Choose one--Do A; or do B" are modal. [CompRules 1999/04/23] The key words "choose one" are important.
G.24.3- If a spell or ability is used to change the target of a modal spell or ability, they cannot change the mode. This may restrict what kinds of targets are legal for the new target. [CompRules 1999/04/23]
G.24.4- If a spell is modal and has different kinds of targets depending on the mode, you choose the mode before picking targets, and you do not pick targets for the unused mode(s). [CompRules 1999/04/23]
G.24.5- Spells which have a specific player "choose one -- " are
also modal. [Rules Team 2001/05/01]
Note- See errata on older cards to determine if they are modal or not.