If you can only test once, I would vary when you test. Testing at the same time each day will give you some info, but not a very clear picture. I would try testing before the morning shot one day, testing for a mid-cycle # another day, testing before the evening shot a different day, etc. A key point would also be the nadir time, when you expect the BG to be at its lowest. I don't know how predictable that is with Lantus though.
I think you will find testing a lot easier than you expect though, once you get the hang of it. I was really scared of it at first, and it took about a month before I got a routine that worked for us. I wasn't on the board yet though, so didn't have the benefit of all the great tips & benefit of experience you can get here.
My best recommendation for learning to test is to run through the procedure without actually testing, so you and your cat get comfortable with the rhythm of it, and with the testing supplies, etc. Hold the lancet device to his ear and click it so he gets used to the noise, hold the meter up there and pretend you are actually testing, etc. I found doing that a few times really helped me get to the point where my hands weren't shaking so much, and I realized he didn't really care that much about having me hold the meter up to his ear, etc. So then when I did actually poke his ear, it wasn't really that big a deal, I don't think he even noticed. Now the whole thing is a piece of cake!
Just post whatever questions you have & there are lots of people here who can guide you through any problem you might be having with testing. It was scary for everyone at first, but heck if I can do it, so can you!!!