Peripheral arterial disease (PAD) prevalence and diabetes mellitus (DM) prevalence are continuously increasing worldwide. The strong relationship between DM and PAD is highlighted by recent evidence. PAD diagnosis in diabetic patients is very important, particularly in patients with diabetic foot disease (DFD); however, it is often made difficult by the characteristics of such diseases. Diagnosing PAD makes it possible to identify patients at a very high cardiovascular risk who require intensive treatment in terms of risk factor modification and medical therapy. The purpose of this review is to discuss the diagnostic methods that allow for a diagnosis of PAD in diabetic patients. Non-invasive tests that address PAD diagnosis will be discussed, such as the ankle-brachial index (ABI), toe pressure (TP), and transcutaneous oxygen pressure (TcPO2). Furthermore, imaging methods, such as duplex ultrasound (DUS), computed tomography angiography (CTA), magnetic resonance angiography (MRA), and digital subtraction angiography (DSA), are described because they allow for diagnosing the anatomical localization and severity of artery stenosis or occlusion in PAD. Non-invasive tests will also be discussed in terms of their ability to assess foot perfusion. Foot perfusion assessment is crucial in the diagnosis of critical limb ischemia (CLI), the most advanced PAD stage, particularly in DFD patients. The impacts of PAD diagnosis and CLI identification in diabetic patients are clinically relevant to prevent amputation and mortality.