That 'old uncle' is quite right.
Let see now...what are some of the properties and functions of a good LSP:
1. Add shine/gloss
2. Make the surface smooth and silky
3. Nice water beading
4. Sheets water when pour onto
5. Dirt/contaminant won't stick easily
6. Provide a temporary barrier against chemical etchings
7. etc......
Rather than trying to find out whether the LSP is still in place or not, consider monitoring whether the LSP is still doing it's job as per your expectations since, even if the LSP is still there, it could already be weak and not doing its job too well anymore. And there's no easy way to do that except for observing and monitoring. For me, the best time for this would be during my routine wash. How easy my wash mitt glide over the surface, how easy dirt being clean, how strong the beading, whether the surface sheets cleanly or any water being 'held' or 'cling' to the surface (by embeded dirt), how smooth my drying towel feels when drying, any water spots that wash/QD cannot remove, etc.......
Or you can forget about the tedious things above and top up with a spray topper every one or two weeks after a wash until you sense that the surface is dirty and in need of a round of 'freshing up'....hehehehe....
Anyway, beading going flat (or diminishing) after a wash doesn't necessarily mean the LSP had been stripped. It could mean the beading from the glossing agent (or wax) left by the shampoo you're using is inferior to the LSP's. Some shampoo may also contain some sort of 'easy rinsing' formula that can make the surface sheet water easier. Don't be surprised if the beading came back during/after a rain when whatever was left by the shampoo had been washed off. I don't think Meg's NXT Wash has the strength to remove any LSP. If it really does, I'd blame the wax instead of the shampoo....

Oh...and don't expect some 'wash & wax' or 'revitalising' shampoo would be able to 'replace' your existing LSP unless it's a 'wet-on-wet' applied topper like Permanon or something. Don't be lazy man...spray wax are everywhere nowadays....hahahaha.....
Suggestion for shampoo: DG 901 Car Wash Concentrate (you can get the smaller bottle to try first, I think the size is around 500ml).
But, if you plan to use a spray topper after one or two washes (or three even), any shampoo from those reliable brands would do just fine...Sonax, Mothers, Soft99, Optimum, 3M, etc....(I'd stay away from CarPlan or Blackfire (?) or those 'al-cheapo' ones though....hehehe....).