Part 1. As @retro says in is reply, you can use a multimeter in series with the load. To do this, you need to interrupt the circuit to the laptop. You could do this either by chopping open the existing adapter cable and breaking one of the two wires, or by making an adapter. Then the multimeter is inserted into the break and used to measure the current. The laptop then needs to be connected and in use.
I see two issues with this though. Firstly, a multimeter is only good at measuring a constant current draw. Laptops nowadays can be extremely dynamic with their load, with short pulses of higher current draw that a multimeter may miss.
Secondly, you need to ensure maximum load - screen at full brightness, processor at 100% with no clock scaling, HD in use, optical drive in use, fans at full speed, battery charging, screen at full brightness, graphics card being pushed etc. It would be easy to miss a corner case and find out that the laptop works 95% of the time.
Part 2. I agree with @retro for most laptops - using an underspecced power supply can cause the power supply to overheat. That said, Lenovo seem to be a little bit more intelligent with how they treat this. I have three Thinkpads and they have been supplied with 90W and 65W power supplies. All three work with both power supplies and are shown in instruction manuals to work with both. The price you pay on two of them is that the battery will not charge as quickly when using the smaller power supply and loading the system. The benefit is that the adapter is much, much smaller. All three detect the connected power supply rated power, and two of them warn when the 65W is attached.
The SL500 seems to be able to work with the 65W adapter:
A colleague's Thinkpad only requires the 65W adapter but was provided with the 90W - I believe that it may be cheaper to produce as it much larger. It certainly gets much less warm, so may be more reliable.
I do not know how the Lenovo power supply rated power is detected by the laptop - there only appear to be two contacts. Some Dell power supplies use a third contact and Dallas Semi serial ID chip to identify themselves.
Overall, I strongly suspect that it will be ok to use this adapter.