Contrary to popular belief, even after a deliberate drug overdose, most conscious patients' histories are fairly accurate. The majority of patients will describe the time of ingestion and the tablets that they have taken accurately. Doses ingested are often more vague as they are frequently described in non-standard dosing units.
This history maybe more unreliable in patients with continuing high suicidal intent, patients in custody and those ingesting illicit drugs. It is also obviously unobtainable when they are unconscious. In these situations, you need collaborative information, which may include contacting friends or relatives at home, talking to the ambulance drivers at the time of arrival and ringing their doctor.
If you have the packet of drugs ingested, it may be possible to estimate the ingested dose by counting the remaining tablets (assuming complete compliance since the time of prescription.) In addition, by ringing the dispensing pharmacist, it is often possible to identify the prescribing doctor(s) as contacts to obtain further information.