Not the best at this but what I would do is

Consider the weightage and allocation of stocks in the ETF
Diversify into different sector for instance (QQQ for tech, any REITS ETF, if it exist, healthcare ETF etc)
Liquidity of the ETF, some ETF is good but it has very little investors buying it thus putting you at a high risk when trying to sell it back to the market.
Past performance. Just because it’s an ETF doesn’t mean it’s good/safer. The past performance of it will give you some insights.
I don’t look at anything else…