I just did a head gasket change on a 1.8L Passat engine and want to make sure everything was done right in it. Already ordered a new timing belt as well but hasn't arrived yet. Can I test the engine's compression with a compression tester by cranking it even without a timing belt installed? The camshafts are also off right now as well so all the valves are closed and I've already turned the crankshaft by hand with no issues.

How is it going to get air in it to compress?

A leakdown tester is what you need for that situation.

you would have to know where the piston is and open the valve at the bottom of the stroke to get air in. either valve will do.
The only catch is figuring out where the crank is since you don't have valves to figure it out.
since you don't have a cam in, you don't have to worry about hitting anything.
or the compression tester should also have a valve to open it to atmo so you should be good too.

Truth