Parish registers do not normally record the date of death, but rather the date of burial, which will be a few days later.
Memorials in a church, or on graves in the churchyard, usually record the date of death. However most people did not leave permanent memorials.
See War memorials and Rolls of Honour
If someone died in Hertfordshire after 30th June 1837 there should be a death certificate. This gives the date and place of death, the name, sex and age of the deceased, their occupation and the cause of death, details of the informant (who may well be a relative) and the date of registration. The only practical way to get a certificate and find out what it says is to purchase a copy (see civil registration). Copies of the General Register Office (GRO) indexes are available worldwide at major genealogical libraries on fiche or microfilm. Hertfordshire certificates are available from HALS, and these are likely to be more accurate than the copies from the GRO.
It should be noted that the information on an English death certificate is limited to name, age, place and cause of death, and name of informant (who may be a relative).
Well-to-do families often printed "In Memorium" cards - which sometimes turn up in family papers.
Obituaries may be available in local papers form the late 19th century (and occasionally earlier) if the individual had some standing in the community. For an example see:
Obituary of James Richardson, Cricketer of St Albans
For the detailed analysis of a Death certificate see SMITH, St Albans, 1942
Page updated July 2009 (link to analysis of death certificate added)