Consult a variety of Spanish-English dictionaries and compare definitions, inclusions and exclusions. Simon and Schuster's is particularly inclusive of regionalisms that may not make it into the other ones; Collins and the Oxford are obviously more current and thus likelier to include new words from both languages.
Spanish Language Dictionaries
Diccionario de la lengua española. 20a. ed. Real Academia Española(Ref. PC 4625 .A3 1984) Also available online (22nd edition) The single most authoritative dictionary of the Spanish language. Emphasis primarily on Castilian, or the Spanish of Spain. Apps available for iPhone and Android phones.
Diccionario panhispánico de dudas. Real Academia Española (Ref. PC 4460 .D5155 2005) Also available online Deals with problems -- common confusions, sources of error, and complexities of the language.
Moliner, María. Diccionario del uso del español (Ref. PC 4625 .M6 1998) Describes proper usage for included terms in Castilian. For the English speaker, good for understanding nuances of usage beyond what is typically found in other dictionaries.
Vox diccionario de uso del español de América y España (Ref. PC 4625 .V69 2004) A dictionary compiled to reflect contemporary Spanish in both Iberian and American sources. Definitions are identified by region or country.
Diccionario ideológico de la lengua española: desde la idea a la palabra; desde la palabra a la idea (Ref. PC 4625 .C3 1959) Includes a thematically organized section which groups words together in conceptual families, like a thesaurus.
Diccionario de mejicanismos (Ref. PC 4832 .S3 1974) Includes words arising from or given distinctive expression in Mexican Spanish.
Jergas de Habla Hispana (Ref. PC 4971 .F57 2006) Also available online. Guide to current slang or argot in Spanish worldwide.
Diccionario de coloquialismos y terminos dialectales del espanol (Ref. PC 4971 .F58 2011)
Los que dicen ¡ay bendito! : dichos, modismos y expresiones del habla coloquial puertorriquena (PC 4854 .P8 N87 2001)
Batchelor, R. E. Using Spanish synonyms (Ref. PC 4591 .B38 1994)
Sainz de Robles, Federico Carlos. Ensayo de un diccionario español de sinónimos y antónimos (Ref. PC4591 .S25 1967) A Spanish-language thesaurus not unlike Roget's in English.
Vox. Diccionario de sinónimos (Ref. PC 4591 .V6 1977)
Corominas, J. Diccionario crítico etimológico de la lengua castellana.  (Ref. PC 4580 .C6) The standard work on the etymology, or derivation and historical development of words in the Spanish language. Emphasis primarily upon Castilian although some words of American origin are included.
Elsevier's concise Spanish etymological dictionary (Ref. PC4580 .G65 1985) More current but not as comprehensive.
McGraw-Hill's Complete Medical Spanish: Practical Spanish for Quick & Confident Communication (PC 4120.M3 R56 2010) Lessons for beginner, intermediate, and advanced audiences on the body, physical examination, interviewing, and treating Spanish-speaking patients.
Spanish for Health Care Professionals (PC 4120.M3 H37 2008) Reviews common clinical terminology and expressions in Spanish, including the body, physical examination, emergency situations, and speaking with pediatric or geriatric patients.