« Reply #23 on: Thursday 13 March 14 10:36 GMT (UK) »
in a word, no, you can't be sure. One way to go about it would be to build up a picture of all the Watkin(s) families in the area with the children belonging to each one and check for burials & marriages for all. One thing you can't do when you get back to parish records is to leap back another generation without a lot of searching to make sure there aren't other possible candidates for the person you're looking for when you're dealing with a common name.
Where did you get the info that William senior was born in Merthyr Tydfil in 1802?
Not all parish records are available online, so you may need to get access to the records at an LDS centre if you can't get to archives.
Cornwall: Allen, Bevan, Bosisto, Carnpezzack, Donithorn, Huddy, James, Retallack, Russell, Vincent, Yeoman
Cards: Thomas (Llanbadarn Fawr)
Glam: Bowler, Cram, Galloway, James, Thomas, Watkins
Lincs: Coupland, Cram
Mon: Cram, Gwyn, John, Philpot, Smart, Watkins
Pembs: Edwards (St. Dogmael's)
Yorks: Airey, Bowler, Elliott, Hare, Hewitt, Kellett, Kemp, Stephenson, Tebb