As others suggested, you can search your wallet address with a block explorer to find all of the assets in the account. I’m a huge fan of blockscan.com - It reads from about a dozen blockchains, so can help find which blockchains you have assets on. (I use the Chainlist.wtf method for adding new ones: https://metamask.zendesk.com/hc/en-us/articles/360043227612-How-to-add-a-custom-network-RPC)
For tokens you can enable the “Token Detection” feature in experimental settings. Its off by default for security. Malicious tokens have been used as an attack vector, so be sure you know what tokens you’re interacting with. You can hide any that you aren’t familiar with by long pressing on mobile, or clicking the three dots on desktop.
If auto detect misses any, you’ll have to manually add them. I prefer the Coingecko method: Adding Custom Tokens to MetaMask Wallet Using CoinGecko