To record the current methylation state of your DNA, you can use bisulfite sequencing. Basically, you take half of your DNA sample and treat with bisulfite, which deaminates cytosines (C->U) , so they read as T instead of C. Methylated cytosines are protected, so they still read as C. You run two sequencing reactions, one with bisulfite-treated and the other with untreated DNA, so you can tell where the true Cs are in the genome. See the below diagram:
Note that if you're thinking of doing this from a longevity point of view, that there are hugely different methylation patterns in different cell types; I'm not sure which cell type you'd want to pick. Also, methylation is not the only covalent modification of DNA that exists, there's also hydroxymethylation, and I'd bet there are more that we haven't discovered yet. There certainly are in bacteria and phage (glucosyl-hydroxymethylcytosine, for one). The hard thing about saving things for the future is that we really don't understand the full picture of what's happening now epigenetically, and as such, we don't have assays to detect things we don't know about.