What is the ruling on the prayer of one who doubts the emission of pre-seminal fluid (madhy) and cannot distinguish between it and urine, and must he check every time?

You are not required to search or inspect for the emission of madhy (pre-seminal fluid). It is sufficient for you to operate on the presumption of no discharge. If you become certain of its emission, either by feeling it or by finding its trace, then you must purify yourself from it and perform ablution (wudu') for prayer. Both madhy and urine are impure (najis) and necessitate the purification of the body and clothes from them, and they both necessitate ablution (wudu') for prayer.