You can always rent one of THESE.
Alternatively you can use a long piece of clear plastic tubing and a reservoir. Like THIS.
You'd put the reservoir on a stand and attach the end of the hose to a 2x at that same height. Then go around to the various places around the patio and make sure there are no high or low spots. Once you get the excavation close, you'd put down stone and compact it, then a 1" layer of sand to set the pavers. You'd screed this off level using level boards on the edges of the patio. Here is a decent tutorial on laying a paver patio. Patio