Can I connect a 12 Volt and 24 Volt AC to DC SMPS to get 36 Volts?
With the supplies unplugged from the mains, measure the resistance between the mains pins (live, nuetral, earth), and the output, if both of them have a near short between earth and the output ground your out of luck, if 1 or none have a connector to the output, you should be fine.

If paranoid, you can test it by having them both powered, and measuring the voltage across the 2 supplies, e.g. supply 1 ground to supply 2 +, if they are isolated they should drift towards 0V within a small amount of time, do the same from supply 1 + to supply 2 ground, if its about the same voltage it reduces to, you can be fairly certain they are isolated and safe to connect in series.