Since you already checked the oil level, I would then check for spark as its the simplest to check and will save you the time of doing other tests if its the problem. if it has spark i would then look at the air box and filter, easy enough to see if there is an issue, if that looked good i would then drain the carb bowl into a clear cup and look for water also drain a little from the take to make sure its flowing properly, if that looks fine, then you would be into something most likely mechanical, so that being said id check the compression.
if it has spark, clean air filter, new gas with no water and good compression, there very well could be an air leak on the intake side or valve lash is wrong. past all that I'm not sure but do check the basics fist and report back.
also as already said you could just spray ether in the air filter, if it starts for a few seconds/minute or runs after the starting fluid that means you have spark, air, compression, so that would lead me to believe it will be a fuel /carb/air leak issue
although ether can mask low compression, so it may start and run using ether but still be too low of compression to pull start without ether.