It is the responsibility of the executor to locate all assets of the decedent's estate, which can be harder than it sounds (also see Taking Inventory).
Common sources of information about asset existence include:
You can also go to your local probate court and have the clerk's office do a search for all records relating to decedent's assets. The results may be helpful if you know little about the estate, but they will likely not be complete, and there may be fees for the service.
MIB (previously known as the Medical Information Bureau) is a non-profit organization that helps life insurance companies with their policy underwriting. MIB covers 99% of the life insurance policies issued in the US and Canada. They offer good general advice on locating policies, and for a small fee an executor can request a search of all policy underwriting activity (this won't tell you if any particular policies are payable, but it will help you in locating them).
It's possible that the deceased had a pension with death benefits. While you may find pension contact information in the Typical Sources above, you can also search for unclaimed pensions for free via the Pension Benefit Guaranty Corporation, a US government agency. The National Registry of Unclaimed Retirement Benefits will separately allow you to also search for unclaimed retirement accounts such as 401Ks. Finally, here's some advice for locating unclaimed pensions in Canada.
When a financial institution loses contact with someone, it is required to notify the government, often turning over any such "abandoned" funds to the state. However, assets typically aren't considered abandoned until 3 years pass with no contact, so depending on when the decedent last contacted the institution, this won't necessarily help you right away.
If you are using EstateExec, its Search for Hidden Assets task will guide you to the appropriate state database. You can also use MissingMoney.com, which may have less coverage than the state databases, but will perform a nationwide search.
When searching these databases, you will need to know any past addresses (at least the city), and alternate names (maiden name, nickname, etc.). These search services can be very finicky, so you'll want to try all sorts of variants on the name, for example filling in the "last name" field with the last name followed by:
You can also hire services that search through public records looking for real estate, business interests, vehicles, and so forth (but not bank accounts, stock investments, and other financial assets). These services are not necessarily cheap, so most executors choose not to use such services unless they believe there is a reasonable likelihood that other assets exist, but cannot locate them.