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"seru" or saseru" are verbs that mean "let "have" & "make" how can you tell the difference?

You tell the difference from the context.

i think seru means " can do "..

Class 1 verb plus せる　　　　　　　　Class ２ verb plusさせる
たとえば：

１田中さんにレボートを書かせます　　　２部長は田中さんを出張させます

seru 　　Probably I think that it is a mistake in writing.
suru I think that I wanted to write it in this way.

suru suru is words to use when the person does something voluntarily.

saseru
　　　　　saseru is words to use when a person is made to do something by a person forcibly.

They both change the verb into "let someone do" form.
There is no difference in the meanings. It's just grammar matter.

"Saseru" can be used attached to other verb or as a stand alone verb (as "suru" can).
"Seru" should be always attached to other verb.

kaku(write) - kakaseru (let someone write)
taberu(eat) - tabesaseru(let someone eat)
suru (do) - saseru (let someone do)
benkyo suru (study) - benkyo saseru (let someone study)

@bill tong
"can do" is "dekiru" =)