assumevalid was introduced in #9484 (Bitcoin Core 0.14.0).
A good summary of IBD, the history of checkpointing and the rational behind assumevalid is available here.
Validating updates is simply running RPC commands, and verifying that the output from your node matches the values being proposed in the pull request.
For example, if you were reviewing #15429:
Run bitcoin-cli getchaintxstats with a block window of 4096, and the blockhash of the new assumevalid block.
From the output, check that the following values match those in the PR:
time-> nTimetxcount-> nTxCounttxrate-> dTxRate
bitcoin-cli getchaintxstats 4096 0000000000000000000f1c54590ee18d15ec70e68c8cd4cfbadb1b4f11697eee
{
"time": 1550374134, 👍
"txcount": 383732546, 👍
"window_final_block_hash": "0000000000000000000f1c54590ee18d15ec70e68c8cd4cfbadb1b4f11697eee",
"window_block_count": 4096,
"window_tx_count": 8872107,
"window_interval": 2407303,
"txrate": 3.685496590998308 👍
}Run bitcoin-cli getblockheader passing the blockhash of the new assumevalid block.
From the output, check that the following values match those in the PR:
chainwork-> consensus.nMinimumChainWorkhash-> consensus.defaultAssumeValidheight-> consensus.defaultAssumeValid comment
bitcoin-cli getblockheader 0000000000000000000f1c54590ee18d15ec70e68c8cd4cfbadb1b4f11697eee
{
"hash": "0000000000000000000f1c54590ee18d15ec70e68c8cd4cfbadb1b4f11697eee", 👍
"confirmations": 3892,
"strippedsize": 115156,
"size": 131899,
"weight": 477367,
"height": 563378, 👍
"version": 536870912,
"versionHex": "20000000",
"merkleroot": "3c9b9295588dbab8388785dd9f70c9d7f42948632556298c8453aad755e30822",
# not displaying tx data
"time": 1550374134,
"mediantime": 1550373349,
"nonce": 3355453177,
"bits": "172e6f88",
"difficulty": 6061518831027.271,
"chainwork": "0000000000000000000000000000000000000000051dc8b82f450202ecb3d471", 👍
"nTx": 359,
"previousblockhash": "00000000000000000001827a07f51769246d852e63c413bc2cd5caa104970f3f",
"nextblockhash": "0000000000000000001d6cf3d8d1bf2e0b1883f6684bb2dddcb57a5d4c81ec7f"