How can I tell if my Mac mini is asleep? I'm have wake issues on my late 2012 Mac mini but one thing I've realised is I don't know when it is going to sleep, to diagnose if it always fails on sleep, or only sometimes. 

During the day the screen will go off and it will wake but after a night, it often won't come back.

How can I tell my Mac has gone to sleep mode in the first place? I've wondered before when seeing the light on my USB mini-hub is still on and the blue light on the external HDD, if it is not sleeping when it should be, since USB power should cease when it's asleep?
I own the same model of Mac mini as you.  I use it without a monitor, so have become quite tuned in to observing when it's in sleep mode and when it's not, which determines whether or not I can connect remotely from my other machine across subnets.

The white light at the front is your indicator.  

When the Mac mini is awake, it maintains constant, full brightness.

When the Mac mini is asleep, the white light begins to "breathe", as its intensity oscillates between dim and full brightness at about the rate of a person breathing at rest.  It's a gradual waxing and waning, but once you notice it, you'll be able to spot it easily.

If you want a slightly more technical option to verify whether or not your Mac mini is awake or asleep, one option is to ping its IP address from another device on your local network.  This assumes that your Mac mini automatically connects to your network upon waking, which is usually the case, but if it doesn't manage to re-establish Wi-Fi networking for any reason, then a ping request will fail even when the machine is awake.  If you don't have another computer to send a ping, there are numerous networking apps available on Android and iOS devices that can do this, and some will even explicitly tell you when a device is awake or asleep by whether or not it registers a network presence.