Is it possible to search availability for upgrades with miles + copay, prior to booking the flight?

All legs from AAA-BBB-CCC-DDD is on UA (W class).

I have enough miles to upgrade and willing to pay the co-pay, but how do I know prior to booking, there is availability in J for those legs for the upgrade? Is this searchable online or do I have to call in?

I don't want to be in a situation where I booked and paid for (W) and wanted to use my miles to upgrade but there is no availability (J)?

Please help.