Sadly there is no way to find out. Unless you have proof nothing can be done, griefing is not tolerated on peaceful survival.
Peaceful survival is the only place where griefing is not accepted, BUT, you have to have video of someone griefing you or else nothing can be done. There is no logblock system currently in place at the moment so unless you have video then you will have to deal with it :(. If you want, I can come on peaceful survival and help you out.
Well, I worked very hard on my claim, (built a farm, mine, house, etc.) and I came on to see it griefed. This makes me very sad especially because of all of the hard work I put in to it. EDIT: What do you get for donating on peaceful survival?
Thanks, thats nice, but I couldn't possibly accept that. What are the perks for peaceful survival for donating?
It's not a big deal. I am very wealthy. If you just come online then I can give you money and items. It truly isn't a big deal.
I am very sorry for your base, but the only way is if you have proof. Like @Chase_16757 and @josebcamacho said.
Unfortunately, the block log system the server used to have is (for whatever reason) no longer in place, therefore there is no easy way to check eh broke what blocks and where.