How would you know if your solution has sodium carbonate or sodium phosphate?

Question

what is a good way to test for these?

Answer 1

To a small sample of the solution, add some acid.  If you see bubbling then it's the carbonate.  The reason is, the reaction between H+ and CO3= produces CO2 gas.  There is no bubbling with H+ and PO4^3-

2H+ + CO3= --> CO2(g)  + H2O(l)