Clearance of the following substance is the hallmark experimental method of measuring the GFR:

Inulin is a polysaccharide starch of about 5-kD molecular weight that is not usually found in the body. It is freely filtered by the glomerulus, but it is not reabsorbed or secreted by the nephron. All the inulin that is filtered flows through the nephron and appears in the urine. Thus, the renal clearance of inulin is relatively large. Inulin in the blood is not taken up by other tissues, and the kidneys are the only excretion route. Thus, inulin clearance is indeed the hallmark experimental method of measuring the GFR. Ref: Eaton D.C., Pooler J.P. (2013). Chapter 3. Clearance. In D.C. Eaton, J.P. Pooler (Eds), Vander's Renal Physiology, 8e.