Here are some quick solutions.

RSK derives its network security properties from merged mining with bitcoin. At the time of writing (Dec 2021), this exceeds 50% of the hashrate of the Bitcoin network. This makes RSK the most secure smart contracts platform in the world.
The easiest way is to check and look for the stats that show the merged mining hashrate, which are available as both an absolute value and as a percentage of the bitcoin network. If you would like to verify this independently, you will need to take a more involved/technical approach: Inspect every single bitcoin block’s coinbase transaction, and identify the percentage of them that contain an RSK tag. With this figure, plus the statistics obtained from the bitcoin network, perform calculations to obtain the merged mining hashrate, both as an absolute value or as a percentage of the bitcoin network.