Apart from the obvious stuff in the solution file, the only give-aways in the project files are the
<Project ToolsVersion="4.0" ....>
which is set to 3.5
in VS2008, and the
<ProductVersion>9.0.30729</ProductVersion>
tag, which doesn't change when you convert the project from VS2008 to VS2010 (well, not at the moment), and the
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v10.0\WebApplications\Microsoft.WebApplication.targets" />
tag which imports web app extension targets (this is set to v9.0
in VS2008, and is only on the web app project).
In any case, i wouldn't be too concerned about which edition of VS to use, apart from the lack of integrated source control and the lack of add-ins in the express version, it'll doo pretty much anything its bigger brothers will.