You can take the plug out and put your finger over the hole, crank the engine and if it pops your finger off you have compression and its not seized. I also look at the oil, smell it does it smell burnt, have gas in it, water, or is it just black and used. Remember there will be a small amount of gas in most small motors. Depending on price, I stay away from motors that have oil that smells burnt. Smell the gas, is it fresh or old, you can tell by the smell in most cases. If its old you will probably need a carb cleaning.
If it was running 18 months ago, and ran when they sat it, it should run again with minimal work.
Then I follow the following to get it running when I get it home.
First I check for compression, I don't have a gauge, I just take the plug off and put my finger over the plug whole, if there's compression it will pop your finger off. If your finger doesn't pop off the whole you may have a valve not seated or a messed up piston. This isn't an accurate test, but it works for me.
Second check to see if there is spark, put the spark plug back into the hole and plug another spark plug into the wire and sit the plug against the engine and crank the engine to see if there is spark. Best to do this in a dark location.
If no spark I check the points, condenser, wires, magnetron, stuff like that until I get a spark.
3rd I check for fuel.
I make sure the fuel lines are clear, then take the carburetor off and clean it out good. If your careful you can reuse the gaskets, I have done this many times.
Get a good manual for the engine you are working on. They should have good trouble shooting guides.
This is how I get 99% of the motors I have gotten to run.
