NeoApo (Japanese) has an "R-15" tag for their anime database. However, it's not that straightforward for anime in general.
At the very least, anime shows can be categorized into:
- TV broadcast
- Premium channel
For works that are screened in the cinema, Eirin is the one that is responsible for the rating, and it has its own database (as already explained in Jimmy's answer).
For the other, Japanese Wikipedia has a non-exhaustive list of age-restricted anime. It is stated that only R15+ and R18+ is given in most case, while G and PG12+ is given only for anime movies. It is also mentioned that TV broadcasts and OVA are generally unrated. Also, since mostly only adult anime (i.e. pornography/hentai) is rated R18+, violence, and graphics anime are not rated as R18+.
Some notable examples:
- Food Wars!: Shokugeki no Soma (G-rated)
- Angel Beats! (G-rated for TV broadcasts, PG12 for Special non-broadcast episodes)
- Jin-Roh: The Wolf Brigade (First movie to be rated PG12)
- Detroit Metal City (First OVA to be rated PG12)
- Senran Kagura (PG12)
- Mardock Scramble The First Compression (PG12 for original movie, R18+ for the full version)
- Mardock Scramble The Second Combustion (G-rated for original movie, R15+ for BD)
- Seitokai Yakuindomo (PG12 for movie, R15+ on AT-X)
- The Island of Giant Insects (PG12 for movie, R15+ for OVA, R15+ on AT-X)
- Strike Witches series (R15+ for BD)
- Interspecies Reviewers (unrated but censored for TV broadcast, R15+ on AT-X)
- VALKYRIE DRIVE -MERMAID- (unrated for TV broadcast, R15+ on AT-X)
- Tsugumomo (unrated for TV broadcast, R15+ for BD)
- Higurashi When They Cry (unrated for TV broadcast, R15+ on AT-X)
- AnimeFesta series (unrated/censored for TV broadcast, R18+ for premium version)
As can be observed, sometimes AT-X rerates some series as R15+ while broadcasting the uncensored version. Their official website also posts news (rarely) and broadcast information (mainly) about age-restricted works on AT-X: