i need to include an if condition which should check for the record type of "assemblyitem", how should i do this?

An if condition in which script type and under what condition?

in userevent script type, if its only assemblyitem, the script should trigger

You should be able to look at scriptContext.newRecord.type. Try outputting that to the script log.

how about to use it in client script?

Look at scriptContext.currentRecord

i used .type, but it did not return anything.

where in your client script? Depending what you're doing you may also need the N/currentrecord module.