Is a slight, involuntary leakage of urine excused, and must one inspect one's underwear before every prayer in this case?

If you are certain that urine has exited, you must repeat the ablution (Wudu') and wash whatever has soiled your clothes. If the spot of impurity is hidden, then wash what you predominantly believe has been affected. A small amount of urine is not excused. One should be wary of obsessive doubts (waswasa). If you feel a drop exiting after performing ablution, disregard it and pay no attention to it, unless you are as certain as the sun, in which case you must wash the affected area and repeat the ablution. As for urinary incontinence (sals al-bawl), it is the continuous discharge of urine without interruption. The person afflicted with it must wash their private parts, bandage them, and perform ablution after the time for prayer has begun.