Flashback - the flashback from a needle in a vein should be dark red with slow flow. Bright red blood that pulsates indicates arterial catheterisation; the needle should be removed and pressure placed for at least five minutes.

Ultrasound - place the sterile probe over the insertion site and examine the vein both parallel and perpendicular to the direction of insertion, looking for presence of the needle / guidewire in the vein (a non-pulsatile, compressible vessel)

Manometry - attach the needle to a transducer system to determine intraluminal pressure.

Blood gas - send a blood sample for analysis. Compare this with the patient's arterial sample or look at indicators of oxygenation in the result.