Rene, your engine has cup shaped lifters that are sized and installed on assembly to achieve the correct clearances. You can check the gaps just like any other rocker clearances but at 50,000 miles I’d think that unnecessary if all seems well and sounds fine and performs alright, I’d suggest all is well. These are durable engines and designed to do huge mileage's although they do sound a little rattly from new.
Each cup will be different and has a stamp on the inside listing it’s size so you check the gaps and write down any difference from the listed clearance size. All exhaust sizes are the same as are the inlet but are different to each other. If you have clearance issues you need to write them down with the correct gap and list the difference for that lifter. When you have checked all inlet and exhaust you have to remove the rocker shafts to access the lifters. If all look to be good just remove the lifters that need changing and check the sizing on that lifter. You can then work out which new lifter you will need to order to bring the gap to a correct size. Often you can only get an adjustment to within a couple of thou so don’t try and go for absolute perfection!
Hope this makes sense, I’m trying to do this on my phone as my computer has refused to work today,

Ps. To remove a lifter use a magnet or a suction cup. A hand valve lapping tool is a good choice!