Your method Checking Payment Gateways are always functioning

We're experiencing fluctuating card transactions on a site, and wondering if anyone has a method/systematic process of making sure the payment gateway is always functioning - twice this year, we've had 7 day downtime of no card transactions. Thankfully we have Paypal, Bitcoin and Banktransfer methods, and customers who call and moan, eventually, so we can attend to it.

But, what do you do to make sure this isn't happening - such is the site with high ticket items, and infrequent transactions, doesn't give me a steady set of conversions to work from.

with thanks