In theory, the primary sequence of a polypeptide will determine the folding of that polypeptide completely. Therefore, an alpha-helical structure should not be able to form a beta-pleated strcuture and vice versa. However, our methods of ab initio protein structure prediction are not yet good enough to be able to fully predict protein structure from primary sequence. Despite the shortcomming in protein structure prediction, there are some general guidelines to differentiate between alpha-helical sequences and beta-sheets.
Since alpha-helices are more compact than beta-sheets, sequences with few bulky groups are more likely to form alpha-helices whereas sequences with many bulky groups are more likely to form beta-sheets. Many adjacent bulky groups or bulky groups spaced 3-4 residues apart (directly above each other or below in the helix) will disfavor helix formation. The same goes with residues of the same charge. Because of the sterics of helices, glycines and prolines disfavor helix formation, although there are notable exceptions (e.g. collagen fibers which consist of mostly glycine and hydroxyproline).
Amphipaticity (spatial separation of hydrophobic and hydrophillic groups) will also favor the formation of secondary-structures. For a helix, spacing hydrophobic groups 3-4 residues apart will create an amphipathic helix, while having hydrophobic residues at every other residue will create an amphipathic beta-sheet.