What gets me for the PA28-161 (I think) is the fact that you can pay 1500 quid and get the MTOW increased. But they don't actually do anything to the plane apart from put a line in the POH that means you always take off with one stage of flap.
The speeds are all the same.
Visual check is the only way to be certain. And it is very common to have flights missed out of tech logs by owners. So looking at the times flown since last refill is not an option.
On the m&B if you work out the worst case situations for full and empty fuel. For different pax loadings and then keep a note of them on your Plog it makes life alot easier. So you can have a max all up pax weight on full tanks max in the back etc. Then you only have to get the graphs out when you go over these limits.
But remember every plane is different so as soon as you start flying a different plane of the same type you need to check that its not got 20kilos of pens stashed in the panel work
Also if you work out the TO and landing for max weight, grass, 5knt tail wind etc. Then you don't need to recompute it unless the RW is less than that distance. Saves playing chicken "well i managed to get it into 300m last week" (with of course a completly different load and conditions)
MJ