If you try to send more SNX than is transferrable the transaction will fail. There are two ways you can check how many SNX are unlocked.
mintr.synthetix.io shows your transferable SNX. To unstake SNX you must burn your Synths or sUSD allow more to be transferable.
A Synthetix team member created a tool for stakers to check. Go to https://codepen.io/justinjmoses/pen/qwqoBR and enter your wallet address, and you'll be told how many unlocked SNX you have.
Follow these instructions:
1) Go to the Synthetix Mainnet contract address (on Etherscan).
2) Go to 'Contract' tab, press 'Read Contract', scroll down to 'transferableSynthetix', enter your wallet address into the input field, and press Query.
3) You'll get a long number back (This is the WEI value that the Ethereum blockchain returns).