The Belbin approach is useful in practice, as it relates to measurement and identification of relevant behaviour. There is considerable evidence that group work will benefit by selecting group members with regard to their preferred Belbin roles (see Cameron, 2002). A very practical activity guide to Belbin is given in O'Sullivan et al. (1996, pp. 59-66).
The alternative options of psychometric testing measure personality and may offer a more sophisticated insight or be longer lasting than behaviour. However, they are generally agreed to be more difficult to relate to relevant group behaviour. The very popular Myers Briggs Type Indication is available online.
Variations between the group requirements for complex and simple tasks is a relevant issue in group selection. Useful considerations of group task congruence are provided by Clark et al. (2001).