Perform simple ABI checks, to determine if an address seems to behave like a Uniswap or Balancer LP pool token.

Call qualifyUniswapLP() or qualifyBalancerLP() to validate an LP token, and add it to the database.

Import the interface file IQualifyLP.sol into a solidity contract, then treat as a simple mapping:

    function do_something_with_lp_token(address lpToken) internal returns (bool) {
	address LPQualAddr = address(0xeE87996440515154fb913Fada7aBB4F2Caecb14f);
	IQualifyLP lpQual = IQualifyLP(LPQualAddr);
	uint256 constant LP_UNISWAP = 1;

	// check QualifyLP's mapping of validated tokens
	uint256 qualResult = lpQual.qualified(lpToken);

	// require that lpToken is a Uniswap-like pair liquidity pool contract
	// Requires someone MUST have previously called qualifyUniswapLP() at some
	// point in the past.
	require(qualResult == LP_UNISWAP);
    }

Ethereum mainnet deployment, v0.9.0: 0xeE87996440515154fb913Fada7aBB4F2Caecb14f

This can only provide a minimum guarantee that the address provided seems to behave like a Uniswap or Balancer pool token. A fork such as SushiSwap, using the same ABI as Uniswap, will pass these qualifications (there is a plan to fix this limitation in the next version, by using the Uniswap Router).