The system dynamics unit also verifies whether the relationship between variables is true or false. The unit of the variable or auxiliary, which is a constant, is free to decide and depends on its variable. Unit determination for Variables in the middle or at the end of the Stock-Flow diagram depends on the form of formulation/equation contained in each variable. "rate", the unit must be per time (year/month/week/day/hours/minute etc).
I would suggest starting by setting meaningful units to stocks (i.e. imitating the real world if possible, for example, a bank account stock should have a money unit like dollars, pounds, etc.). Use the same method for the auxiliary variables that have real-life counterparts or you are so sure about how they are measured.
The next step is to give units to flows, which will be the units of the stocks divided by the unit of the time (e.g. dollars/year).
For the rest of the auxiliary variables, give them units that will make them in correct relationships with the other variables connected to them.
In Vensim PLE, you can select Model>Units Check from the menu. You will either get 'Units are OK.' or a detailed unit errors list.