Generally what I do is look around to see if there is anyone old enough to be the parents. In this case, there are, in the 1851 census of St. Andrews Parish, Charlotte Co., NB. See:
http://automatedgenealogy.com/censusnb51/View.jsp?id=3736http://automatedgenealogy.com/censusnb51/View.jsp?id=3762A problem is that Thomas and Jane Alexander stated that they arrived in 1822. Robert stated that he was born in NB. This may not be accurate and I suspect that it is not.
Arnold Krause at:
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&d...seems to share that suspicion as well and makes Robert the s/o Thomas and Jane. His work is extensive and pretty good, and he references a baptism. The assumption may not be conclusive, but seems to have a high probability. As Arnold E. Krause points out, more evidence would be useful. This may be in the disposition of Thomas' estate. Did he leave a will? Was his land divided or sold? The evidence may exist, but not be obvious.
Thomas and Jane are still in Charlotte Co. in 1861:
http://data2.collectionscanada.gc.ca/1861/jpg/4108534_00270....Also in 1871:
http://data2.collectionscanada.ca/1871/jpg/4396592_00193.jpgJane is a widow by 1881:
http://data2.collectionscanada.gc.ca/e/e326/e008130374.jpgThomas.