I senior developer should be able to me what the difference between value and reference types are, talk to me about thread safety etc.
If you tell me you are a senior C# developer and you cannot tell me whether the static constructor or instance constructor executes first or what a sealed class is, or
if you are senior Java developer and you don't know the super class' constructor will be called implicitly by the subclass,
or
inner join vs left join, pros and cons of an index on a database table
In fact, just last week I had a guy with 10+ years experience tell me he can't give me textbook answers when I asked him the difference between value and reference types. I didn't even bother asking him if he knew what a stackoverflow exception is.
Point is, if you consider yourself a senior developer and you cannot answer questions like the above or at least try and get close then what type of senior developer are you really? A Visual Studio Developer? All those questions are relevant in your daily coding life. There's nothing "text book" about them.
No offence to anybody here but the struggle to find people worth the money they ask is real.