Turn off the electricity to the furnace. Hold the knob/handle on the furnace's access door and pull it up, then out, to remove it.
Turn the electricity to the furnace back on. If the ignitor flame comes on within a few seconds but is small and orange/red in color, the ignitor needs to be replaced. Also, if there is a smell of gas after the flame has come on, it needs replacing.
Turn the electricity to the furnace back off, and then back on. This time, see how long it takes for the ignitor to ignite, by looking at your watch. If it takes two minutes or more, and even if the flame is healthy and there's no smell of gas, the ignitor needs to be replaced. However, if the flame is healthy and takes only a few seconds to come on, the ignitor is fine. In this case, replace the furnace's access door by pushing it into position, and slotting it down into place.