One way would be to ask if you can describe any overt evidence that we should expect to see if there were a designer
As you say (I think!), it must be the case that in any universe that has conscious minds it must seem to those minds that the universe has at least the superficial appearance of being designed.
I find it useful to think that if our universe was designed, it was designed by something that either didn't want the inhabitants to know who designed it and what for, or simply didn't care. It the creator wanted us to know, it would have left its TM on the mechanism for all of us to find, no matter how primitive we are.
So at the very most, the designer god, if he did exist, has nothing to do with the religious god that people here talk about.
There are no new observed novel designs. Even if information was gained, they have yet to show an improvement to a fruit-fly. We see adaptations, but the same essential organisms.
You may be interested in this:
Scientists report on the development of engineered silkworms that express a synthetic spider silk protein and stably produce chimeric silk fibers that are stronger than parental silkworm silk fibers and as tough as spider dragline silk
Spiders make extremely strong silk, but it can't be farmed (spiders eat each other). Silkworms can easily be farmed but make weak silk. So geneticists stitch a spider silk gene into a silk moth and get super strong silk that can be farmed.
Another example from my favourite snail man, Steve Jones.
LET'S TURN TO THE second issue: natural selection. People often think of natural selection as something almost magical. But it isn't. It's extraordinarily simple. I first witnessed natural selection taking place in a soap factory in Liverpool in the 1960s, where I worked after leaving school.
Detergent was made then as it is made now: by forcing boiling hot chemicals at great pressure through a nozzle. As the mixture zooms out, the pressure drops, and it breaks into a vapour that is sucked away and a powder which is then sold as detergent.
The nozzles were a damn nuisance. They were inefficient, kept blocking and made detergent grains of different sizes.
Unilever and various other companies hired mathematicians and physicists in an attempt to improve the situation. But they didn't do very well; it turns out that the physics and maths of the transition from liquid to powder is quite difficult to understand.
So, almost in despair, they turned to the lowly biologists and asked if they had anything to add. What the biologists did was to apply Darwinian natural selection.
They made 10 copies of the nozzles, with slight changes absolutely at random. Some nozzles were longer, some shorter, some had a bigger or smaller hole, maybe a few grooves on the inside. But one of them improved a very small amount on the original, perhaps by just one or two per cent.
Based on the improved nozzle, they made another 10 slightly different copies, and repeated the process. After only 45 generations – which would be an utterly trivial instant in evolutionary time – they had a nozzle that worked many times better than the original. This was without any forethought of any kind, only by a simple application of evolutionary mechanisms.