Find if an account has opted in to receive an asset

1

Is there a way to find out if an account has already opted in to receive an asset? Otherwise, we will need to maintain this information in our application to prevent sending 0 value transactions multiple times to opt in.

2

You can use the algod REST API or any SDK to do it.
Here is an example with the algoexplorer API endpoint:

$ curl -s https://algoexplorerapi.io/v2/accounts/UK2GUCW2K4V3BCJU5EIPQYX37I7K4LRKM6KSHVWKT3QD2W6JHMWI3H6U2Q | jq
{
  "address": "UK2GUCW2K4V3BCJU5EIPQYX37I7K4LRKM6KSHVWKT3QD2W6JHMWI3H6U2Q",
  "amount": 200000,
  "amount-without-pending-rewards": 200000,
  "apps-local-state": [],
  "apps-total-schema": {
    "num-byte-slice": 0,
    "num-uint": 0
  },
  "assets": [
    {
      "amount": 15256789,
      "asset-id": 135464366,
      "creator": "ABKFIENZR4YXOTTWTQCOJ5NNG4CXGRNTR3VMHYNDHE5UQZ6Y2DSCIY772U",
      "is-frozen": false
    }
  ],
  "created-apps": [],
  "created-assets": [],
  "pending-rewards": 0,
  "reward-base": 182866,
  "rewards": 0,
  "round": 13591552,
  "status": "Offline"
}

The list of assets the account is opted in is assets.